1. Put bluntly, fiction by the fans. Created by fans of any particular fandom, 50% of fanfiction is crap, 24% is non-crappy smut
, 25% fits into the category of crap and smut, and the remaining 1% is some pretty good stuff.
2. Something English teachers don't like.
1. Trying to find a non-crap, non-smut fanfiction is a little like trying to mine for gold.
2. My friend's Star Wars fanfiction got an F in English class simply because it was fanfiction.