Flat colours effect?

I'm sorry, I really didn't know what this kind of an effect would be called... I've been trying to find a tutorial for this because I've been wanting to learn how to do this for a long time. Here are some examples made by catgirlrach:



And a link to a wallpaper.

Can someone tell me how to do that please? I use Photoshop7, by the way. Thanks so much, I'd really appreciate it.