Okay, so you wanna know what fanfic is.
Well, it's actually quite simple. Fanfic is short for fanfiction. Fanfiction is just what it sounds like: fiction written by fans. Fanfiction is written for just about everything under the sun- tv shows, books, comics, cartoons, music, whatever. It's pretty cool, actually. Anybody can write fanfic, and just about anybody can get it on the net. The only thing you have to watch out for is copyrights.
Copyrights are those little c's inside circles that are found on...oh, just about everything. Actually, copyrights are much more complicated.
"copyright [as defined in The New Lexicon Webster's Dictionary of the English Language]- (1) n. the exclusive right to reproduce literary, dramatic, artistic, or musical work, given by law for a certain period to an author etc. or his [or her] agent [...] (2) v.t. to secure a copyright on (a book, film, etc.) (3)
adj. protected by copyright"
Basically, it just means that you run the risk of getting sued if you write fanfic and put it on the net. That's why you always have to include disclaimers before your fic. Most authors don't do that very formally, merely because they either
a) don't know who the real owners are (possibly, "anymore"), or
b) they don't want to be bothered.
All you really have to do is say something like, "Lois Lane, Clark Kent, Superman, etc. don't belong to me." That's basically it. It's great if you want to elaborate; please do, if you can. You get used to it after a while. Plus, you can try to challenge yourself by saying the above in as weird or different a way as possible, but still be accurate.
Let me just say that you should always be bothered. In my eyes, it's not only safer to put disclaimers, it's also the right thing. Because, if you don't, you're essentially stealing. You can say that the actual words you put down on paper (or screen) belong to you (as long as they really do), but if you're using someone else's characters, ideas, etc., you should acknowledge that those characters etc. don't belong to you. After all, you wouldn't want anyone to take your stuff without asking, right? So just be a little considerate, and acknowledge that someone else is a little better than you and put disclaimers.