Here is a shot of the different series of camras that Canon has to offer:
There's several models of Rebel, then there's the 50D, the 5D, and the 1Ds.
Broadly, which one is for whom broadly?
The Canon Rebel or EOS XXXXD Series
Beginner/Intermediate models for hobby photographers who are transitioning to dSLR's, and looking to start, but within a moderate budget. The difference between the different Rebel cameras you've listed are just subsequent upgrades, and features: more megapixels, HD video, faster burst rate, etc. These camera's are quite capable of producing amazing pictures. What they lack in comparison to the higher priced line-ups are typically features and options. Much like you would expect when going from a base-model car to it's fancier deluxe model. These camera's use an APS-C sensor, which results in a 1.6x Field of View crop when compared to a 35mm film negative. A 100mm lens when used with an APS-C camera will result in a Field of View that is equivalent to 160mm on a 35mm film camera. These bodies make for excellent choices if you are on a budget, new to photography, or not sure how deep you want to get into the world of SLR photography (you aint seen nothing yet, alice).
The Canon 60D or XXD Series
This is what has been dubbed the prosumer model. Taking all the features of the Rebel series and adding many more options, to create a camera that suits well for a beginner, but enough features and options that advanced photographers can make use of, resulting in a body that will "grow" with you as you advance in your skills. These camera's have a more advanced AF system compared to the Rebel series, have a high burst shot count, making them suitable for action photography, but lack in many of the features found in Canon's pro line. These camera's make for excellent bodies for someone who has experience with SLR photography, or new comer who is serious about getting into photography. For many pro's, these bodies serve as a 3rd body backup (some even using them as 2nd body backups). These bodies are also APS-C bodies, so any lens paired with the body will have a 1.6 FOV crop.
The Canon 5D Mark II/5D Mark III
This is Canon's non-professional line Full Frame camera. Unlike the Rebel and XXD series, the 5D Mark II and 5D Mark III have a full frame image sensor. Thus there is no Field of View Crop. They are designed for studio and landscape work, primarily because of the slower burst rate, and AF system. These are more specialized bodies; they are a step up in terms of both features, and image quality from the XXD line, but lack in the high-speed performance that the XXD offers for sports.
The Canon 6D
The 6D is Canon's newest and least expensive DSLR sporting a full frame sensor. Unlike the 5D Mark III, the 6D has a cropped viewfinder, allowing the photographer to see only 97% of the frame before shooting. Notable features include built-in WiFi, GPS, HD video, and build quality similar to that of the 5D and 7D.
The Canon 7D
What The 5D Mark II is for studio and landscape photography, the 7D is for sports, wildlife and action shots. This is a specialized camera with an AF system, and high burst rate designed to offer sports/action shooters a lot more performance. The 7D is an APS-C 1.6x crop camera, like the Rebel and XXD series.
The Canon 1 Series
This is Canon's professional camera line. These are built with the tough demands of professionals in mind. I have seen hobbyist shooters with 1 series bodies (and certainly if money was no object I would have a closet full of 'em). It sports an insane AF system, a tank-like build quality, and a fine degree of control and features that allow users to custom tailor the body for their needs. The bodies are full weather sealed, when paired with a weathersealed lens. The shutters have a very high life expectancy, and it's very common to see used 1 series bodies with shutter actuation counts in the 200,000 range. Canon has two types of 1 Series bodies, each specialized for specific needs:
The Canon 1D Series is designed for the sports/action/wildlife shooter with a very high burst rate, and a highly sensitive AF system tuned to keep the focus on the action. This body is a 1.3x crop factor.
The Canon 1Ds Series is a full framed body ideal for studio and landscape work. It has a very high megapixel count, and has tonal range suitable for studio work.
Now, that isn't to say you can't use the 1Ds for sports, or the 1D for studio work. Either will work, just that the 1D has more features specific to action, compared to the 1Ds.
That also goes to say that you couldn't be a professional photographer using Rebel bodies. All these camera's, when used properly, are capable of producing AMAZING images. It's more that the Canon 1 Series is designed to be used abused in a daily basis, where as the Rebel is not.
Canon have different names for cameras in different parts of the world, so let's start with a translation table for those:
EOS 1100D = EOS Rebel T3 = EOS Kiss X50 EOS 1000D = EOS Rebel XS = EOS Kiss F EOS 600D = EOS Rebel T3i = EOS Kiss X5 EOS 550D = EOS Rebel T2i = EOS Kiss X4 EOS 500D = EOS Rebel T1i = EOS Kiss X3 EOS 450D = EOS Rebel XSi = EOS Kiss X2 EOS 400D = EOS Rebel XTi = EOS Kiss X EOS 350D = EOS Rebel XT = EOS Kiss N EOS 300D = EOS Rebel = EOS Kiss
The numbering system on the left is intended to tell you how advanced the cameras are. EOS 1 is the professional end, and EOS 1000 is the least feature packed model.
So, generally, the lower the number, the more expensive the camera is. However, a newer camera in the same segment gets a higher number, so 550D is the replacement for 500D.
Canon has an odd numbering system and they adjusted it over the years too, so it not entirely consistent (no manufacturer is but Canon is one of the bigger offenders).
All Canon DSLRs actually have a model number which is used in Europe. In America these are replaced by letters such as XS, XSi T3i, etc. Asia uses a different naming system (Kiss). For example, the Canon Rebel XS is the 1000D. If you use the model numbers, things are a little easier to sort out:
The a suffix which was only used twice means for Astrophotography.
Broadly, 4-digit models are the absolute basic models with very limited feature sets and interface. 3-digit models are only a tad better with a few more functions and a slightly larger LCD and maybe a few more megapixels.
Two-digit models are mid-range. They share the same sensor with the 3-digit models but have better bodies with more controls and - crucially - dual control-dials. This makes them more efficient to use. The also shoot noticeably faster.
The 7D is the top-end crop-sensor model, sharing the same sensor with the 60D, but offering a weather-sealed body and a 100% coverage viewfinder, which makes it much more suitable for serious use. It also shoots at an even faster rate.
The 5D series is a full-frame model which looks at lot like a 7D but offers improved image quality. The latest is the 5D Mark III which I just reviewed last week. It has an ultra-fast autofocus system which is also shared with the top-end 1D series.
The 1D X is the top of the line in terms of image quality and speed. It has less resolution than the 7D or 5D but higher sensitivity to light and much faster speed. It is also built like a tank with an integrated vertical grip and huge battery.
The Canon range is loosely in ranges aimed at entry-level (the Rebel series in North America, and Kiss series in Japan, and three or four digit model numbers elsewhere), "Prosumer" (those with 2 digit model numbers, such as the 50D) and professional (with single digit model numbers such as the 5D)
At any one time, there may be more than one model in a particular range, with a more specific target audience; for example, the 1Ds series is aimed at studio photography, with a large sensor; but the 1D (with no s) is more tuned towards sports photography, with the ability to do longer bursts of photos.
At the opposite end of the scale, there are cheaper options for those on a tighter budget - as an example, the 1000D/Rebel XS/Kiss F uses the older Dig!c 3 processor as well as having a lower resolution sensor than the other Rebels.
As a rule of thumb, higher specs get higher numbers except in the single digit range, when the lower numbers are considered better.
Alan has alread covered the general differences between the ranges in his answer but here's some more detail about the lower end models.
This Wikipedia table ("camera template"):
is always up-to-date, and lists the models by generation and tier, as well as gives information on the processor generation used in each model.
There are similar tables for other brands/types of cameras under the Wikipedia camera templates listing.
Generally speaking, the XXXXD bodies are using an older XXXD model's hardware with a newer processor and sensor. The XXXD bodies are entry-level crop with only a single control wheel and no top-plate LCD (until the 760D was introduced with these two features). The XXD bodies are prosumer crop bodies, with dual control wheels and a top-plate LCD to help see/change settings without menu diving. The 7D bodies are prosumer crop bodies with more hardware controls and fast-action features (higher frame rate, dual processors). The 6D is a sort of full-frame XXD; the 5DMkIII is a sort of full-frame 7D. The 1DX line are the full frame professional bodies with integrated portrait grip, top of the line autofocus, and weathersealing.
The main thing to keep in mind is that typically, bodies with the same format and same processor generation share the same sensor and processor; so image quality differences will be minimal or non-existent; e.g., the 7D, 60D, and 600D are all Digic 4 cameras with 18MP sensors. The main differences are in the hardware UI (buttons, dials, joysticks, etc.) and other usability features (flash master in the pop-up, menu selections, white balancing by Kelvins, etc.).
The larger the number within a tier, typically the newer the generation (e.g., 10D, 20D, 30D, 40D, 50D, 60D, 70D). The newer the generation and the higher the tier, the more expensive the camera gets.
However, because killing engineers is a bad idea when you're a technology company :), the different tiers do not upgrade from generation simultaneously, so some cameras from lower tiers can move ahead a processor generation before higher-tiered models. Typically, sensor and generation advances do "trickle down" but the lower the tier, the faster the refresh cycle. dRebels, for example, tend to get a new model once a year, while the 1DX, 5D, and 7D models may take three or more years between refreshes.
One more note: camera prices depreciate, even while the camera is new, so a model that's been out longer will have fallen farther from its initial MSRP than a newer model, which is why sometimes you'll see new lower-tier cameras that look as if they're close in price to a higher-tiered one (faster refresh); like the current price on a new 70D body ($1000) vs. a new 7D ($1300). But if a 7DMkII comes out, it won't be priced the same, but will be closer to the initial MSRP of the 7D [$1700 in late 2009, body only] (if not higher).
Above, you can see a screenshot of Wikipedia's EOS digital timeline. Colours indicate DSP generation, x-axis is time (from past to present), y-axis is level (from entry to pro) with exceptions, such as the 1D C, and of course only within the camera types and sensor sizes.
From 2003 to Q1/2019, the EOS naming scheme we have in Europe is: (I never got the hang of Kiss or Rebel, so please forgive me for not talking about that. Also, I left out pre-2003 because adding another exception just for the D30 and D60 seemed unreasonable.)
In general: the less digits, the more pro-grade the camera. All except the 1-digit series offer APS-C sensors.
1000D -> 1100D -> 1200D -> ...)
400D -> 450D -> 500D -> ...)
40D -> 50D -> 60D -> ...)
Since there are not too many on the market as of now, this part could need amendment at each new release.
All M-Series offer APS-C.
Depending on your location, that list is slightly out of date. IIRC, the XS was dropped, and the 3 'Rebel' cameras are the T2i, T1i, and XSi.
With a budget of ~1000 USD, the 50D is slightly above your price range, and the T2i, slightly below.
The 50D will be a slightly nicer, more professional camera than the T2i. As mentioned above, its more of the prosumer line.
One of the big things upcoming in DSLR's are HD video recording. I personally have the T1i, and love it. The video recording updates on the T2i make this an even better camera at that price. I consider myself just shy of a prosumer when it comes to cameras. Its a hobby and I have done some photo shoots for others, but mostly I do it for fun. And the extra benefit of HD video is nice.
If video on a DSLR is of interest to you - go with the T2i and buy the ~ $100 Canon 50mm 1.8 (an very cool lens). You can sometimes find it around $80 as well. If not, get the 50D, and probably get the 50mm lens anyways :)
There are a number of "families" or groups of Canon DSLRs, and within each group a fairly consistent naming/numbering system is used.
The Rebels, for example, are considered "entry-level" and in addition to the "Rebel whatever" name also have a numeric designation: 300D (Rebel), 350D (Rebel XT), 400D (XTi), 450D (XSi), 500D (T1i), 550D (T2i), 600D (T3i). You can see the consistent xxxD numbering.
"Mid-range" Canon DSLRs currently use xxD numbering (they were originally Dxx, though): 10D, 20D, 30D, 40D, 50D, 60D. The 20Da and 60Da are similar in features to their non-a versions, but have different sensors for astrophotography.
Wikipedia has a nice summary of all the different Canon models at the bottom of this page.