What is the definition of "channel" in photography? I know that there are "RGB channels", but what does that actually mean?
In the context of a digital image, a channel is simply an array of values, one per pixel, that together specify one aspect or dimension of the image. For example, you can describe an image by specifying the red, green, and blue values for each of the pixels in the image. This is known as RGB (for red, green, blue). An RGB image has three channels. The red channel is an array of values that specifies the intensity of red for each pixel in the image. The green and blue channels similarly specify the intensity of green and blue, respectively, for each pixel. When the three channels are combined, you get a full color image.
So, let say we've got a photo of a duck. As I said above, any given channel represents only one component of the image information, so we can visualize a channel as a grayscale image by interpreting the channel as indicating intensity. Here's what the red channel looks like:
Because this is the red channel, a white pixel in this image means that the pixel will have the maximum amount of red in the final photo. A black pixel has zero intensity, so there will be no red in the corresponding pixel in the final image. Here are the green and blue channels:
When the channels are interpreted together as red, green, and blue you get the full color image:
There are other color models beside RGB, such as the HSV (hue, saturation, value) and CMYK (cyan, magenta, yellow, black) models. These also use channels to represent the various components of each pixel. And channels can be used to represent other kinds of per-pixel information, too. For example, RGBA images are just like the RGB image above, except that they have a fourth channel that indicates the alpha or transparency value for each pixel.
Channels are based on the color mode you're working and viewing in. Each color mode works differently. For example a CMYK Printer has 4 heads - Cyan, Magenta, Yellow, and Key. In the physical world those are considered 4 inks but in the digital world those are called Channels.
The thing to really remember is not all color modes work the same way in how they combine these channels. The two most common for example are opposites. RGB is additive but CMYK is subtractive. But, what does this mean in terms of this question?
Well when you look at a White document in CMYK the 4 channels will be empty.
Now convert that plain White document to RGB and instead all 3 channels (Red, Green and Blue) will be full. (In Photoshop Preferences → Interface you can have the channels show in their color. I don't usually use this and really only turned it on for this screenshot)
When starting out think of each channel as its own Grayscale Image, but remember RGB the darker it is the bright the color while in CMYK the darker it is the darker the image. This is because RGB is adding light, while CMYK is imitating how inks are added together.
Where it gets trickier is in thinking about it. While technically speaking CMYK is Subtractive and RGB is Additive, for the intent of design it can often be easier to come with terms of how channels work thinking exactly the opposite.
Here we have a Gray Image, 119, 119, 119 in the RGB space. Our channels are all equally distributed. When I push more Blue into the square, the Blue channel gets Lighter not Darker. For some people getting lighter is easier to think of as a subtraction even though in RGB that's actually an addition. Complicated I know.
See I added Blue, and the Blue channel got lighter not darker.
My advice is to start working with channels more and you'll get better at it. Go into the individual channels and make changes to see what happens to your image. You might also want to get the book Photoshop Channel Chops.
Channel... Well this is veeeery simple but can get really compicated so I am starting with the easy one.
A) Lucky for us, our eyes can be fooled using just some promary colors. In tis case RGB: Red, Green, Blue, colors.
B) A computer saves the information with 0 and 1. This is binary code.
(There are 10 kinds of people, thoose who understand binary and thoose who not).
You can store values with thoose bits, for example from 0-255. 0 for the darkest luminosity level, 255 for the brightest.
C) On a digital image every point that forms part of it is called a pixel.
Join A+B+C and you have a file with information for the image. The information of the brightness of every pixel for each color Red, Green and Blue.
A channel is the isolated information for each color. One Red channel is the same as a grayscale image of the Red component.
The complex answer is probably too complex
Because there are many diferent types of color models each with their own unique channels: CMYK, RGBA (RGB with transparency), Yuv, Lab, HSB, direct inks, etc, But they are all diferent ways of describing the intensity of its component as a grayscale image.
To simulate the human vision (what is required to record and transfer colour images) any device needs at least three types of measurements for each pixel/dot of the image: three different layers for film, thee different layers for Foveon, three different filters for Bayer sensors, etc. The image storing/transferring may be much more complex: there are colour models named Lab, Luv, HSB and much more others. Same image stored in different colour models and spaces will yield different channels. The term "channel" refers to the array of measurements of same kind, i.e.
Lab which means
Luminousity is a channel.
RGB channels is an ambigous wording and oftenly denotes the channels of an image stored in sRGB colour space.
Working with individual channels in image editors is needed to accomplish specific editing results. Viewing channel data is important for studying images for different purposes.
Digital photography (and its preceding, or related technologies such as Color TV) have follow a RGB color model for a reason: the "Trichromatic Color Vision" of the Human Race
The Human retina has 3 types of cells that act as sensors for some bands (-"channels") of the electromagnetic spectrum. Many organisms have such cells that are particularly sensitive for the wavelengths that correspond to the Red, Green and Blue colors of the Rainbow.
(Why has the eye evolved to become sensitive to these parts of the EM spectrum? - this is related to characteristics of the Sun's light, and its absorption in the Earth's atmosphere.)
If all 3 cell types are stimulated equally to the max, the human brain registers this as "white".
Ultimately, you can substitute the word "channels" with "bands", "frequency ranges", or "well-defined parts of the Electromagnetic spectrum".