"codec" is short for "compressor-decompressor" and refers to the small program that does the work of compressing the file size when storing the movie to disk, and decompressing during playback. It's fairly common for the programs to be called a codec regardless of whether they're for mac or windows.

If you're going to be changing the format of your images, it's really going to be better if you don't use any compression at all, until you're ready to distribute the final result. Most compression schemes lose a small amount of data that was in the original; as you save repeatedly using different methods, you lose a little more each time, resulting in graininess.

There are some compression schemes that don't result in the loss of data, and they're nice for intermediate work. One I really like is called Lagarith, and you can get it for Windows free at http://lags.leetcode.net/codec.html.

There's no "best" codec for Poser animations. If you don't have very complex materials in your scene, and you have large areas that are the same color, you'll get good results with Lagarith. If you're going to distribute it to other people, I think DivX is a good choice; the codec is also free, and you have a lot of control over the quality. If you're going to send it out for streaming, to YouTube for example, I would avoid compression altogether; they have a 2 Gb file size limit, and sending your video uncompressed works a lot better with them.