In the context of
anime artwork, CG reffers drawings that have been done on paper then
scanned in, and enhanced using
Photoshop (or some other image manipulation package) and a
graphics tablet. Lines are cleaned up, and
airbrushed fills and effects are added, with frequently stunning results.
Subject matter is often often, but not restricted to, characters from anime or video games. Such art is usually labled Chara or Game. Lot's of original character designs are also to be found however.
Some CG can be found on english art pages, sometimes displayed without the permission of the original artist, but the best place to find CG is Japanese web pages. Since most of them are Japanese only, Your best bet of finding what you're after is to use an english index of such pages, or a search engine like Surfer's Paradise. Be careful though, because there's quite a lot of stuff out there that you probably don't want to see.