A diffuse is the main map which the details are on.
Oh, and just so you know, just because skinning is easy, it may not be your thing. There are multiple other aspects to modding such as modelling, animating and more depending on which engine you mod. Although, never try to do all at once, you'll have low quality in all aspects. Only some people can do so. You can try other aspects once you're good enough on one.
Good luck, there's a tutorial section to look for help, the forums and the multiple skilled members of this site.