Like pfelberg says, you need to take in mind every RGB image is made of three 8bit images, and Alpha is another 8bit image used to determine transparency by his gray value (white will be solid and black transparent, or the inverse). Some formats and containers have the ability to include an alpha channel (you will see this named as RGBA), but that is just a particular thing. Alpha channel can be included in the same video file, taked by another file, or just created on the fly. Any time you see a filter that have in his name the words Key, Matte, Mask, Choke, are telling you that will work on the alpha channel. So, a lumakey will make an alpha channel from luminance values of certain image, chromakey will do it by a defined range of color, etc. And you will see too, that not all chromakeyers work in the same way, some work over a hue range, some over color-luma-sat range, some by difference and substraction techniques..
Is out there several ways to achieve what you want, and that's just great! because the results are not the same across different techniques. According to the footage and what you wanna get rid of, you will find some processes are better than others.
One time you figure out that, you undestand the process anywhere, even when someone haved the idea of calling it rejector
