How can I use these prime lenses for Canon T2I? Are these cheap and good? I have no clue what M42 lenses are. Please post appropriate links for the lenses and adapter that I can use for my canon t2i.
M42 prime lenses are single focal length lenses that fit the M42 mount, also known as screwmount. There are many M42 bodies other than the Zeiss Contax, but no digital M42 bodies.
Beware that Contax mount lenses are typically not M42 mount, but it's replacement, the C/Y mount or even N-mount.
The reason why primes are recommended is because when these were made, zoom lenses were of particularly poor quality.
Any EF-m42 adaptor should work just fine. I am not positive, but some may result in the aperture and distance markings to be at 6 o'clock instead of 12.
You will have manual focus and manual aperture control (you adjust the aperture ring and the lens stops down). Some bodies will allow more or less functions like metering, Av mode, focus-trap, IS.
Some lenses will have an Auto/Manual switch or have preset rings, which can be very helpful. In Auto (preset ring set one way), it will be wide open, allowing you to compose, focus, etc. Then switch to Manual and the lens will stop down to whatever the Aperture ring is set to. Then you shoot. Very helpful if you are using extension tubes and doing macros.
While there are tons of great M42 primes, there are also a lot of bad ones. Look up reviews in the vintage section here.
Just to add to jrista's point, the M42 mount lenses can often be found quite cheaply and many of them are extremely good, especially ones made by Zeiss. Pentax popularized this mount enourmously in its day, so there is also usually a lot of Pentax lenses available in this mount, also quite good. Which reminds me, you may sometimes see them referred to as "Pentax screw/thread mount" in adverts as a result of the popularization, however it's M42.
Anyways, for reviews and other good info on manual lenses, the Manual Focus Lenses site has a bunch of info and reviews, though not all lenses are M42.
Finally, bear in mind, these lenses will be fully manual and so you need to use them in manual mode on your T2i, possibly including metering (I'm not sure how to do stop-down metering on the Canon), and definitely including focusing.
The M42 is a standard lens mount. It has a 42mm diameter, hence the name. It was apparently originated by Zeiss, however it appears to have extremely broad compatibility with both old and obsolete as well as modern and current camera bodies. This includes Canon EOS, such as a T2i.
It should be noted that to use this mount on camera bodies other than the Zeiss Contax, you will likely require a mounting adapter. Additionally, these lenses do not include any electronic capabilities (or the adapter does not support electronic adaptation), so you will not have any features like auto-focus or electronic aperture control. Everything would be manual if you used one of these lenses on a modern electronic camera.
You can read more about this mount and its lenses at wikipedia.