I don't personally recommend anyone doing nude modeling even though it usually pays the most under freelance circumstances. If you do decide to do it you have to be 18 or older.
Artistic: Usually a black and white photo, artistic poses. Something you'd see in an art book and not on a porn site. Click here to see art nudes
Topless: Only showing the breasts. Poses can be basic, artistic, or erotic.
Body Paint: The best way I can describe this is that paint is your "outfit" so you're not really "nude" as such.
Click here to see body paint
Erotic/Fantasy: Something you would expect to see posted on a porn site.
Implied: model can sometimes get away with wearing clothes during implied shots; pictures are portrayed as the model not wearing anything. If clothes are worn, they will be hidden.
Click here to see implied nudes
Basic Nudity: Posing like you would normally, only you would be nude. Typically just standing in front of the camera... not particularly erotic but not artistic either.
Obviously, there are some loop holes in some of these. A lot of people don't realize that you don't have to be nude to be in a porn film, also, you can be fully clothed and still do erotic shots. I cannot stress enough to another model that it's important to be careful about the style of images you take.