Where it all began for me.
I discovered fanfic on the internet of a community college. The first two fandoms were The X-Files (to date myself) and The X-Men (largely from the animated show). A vague awareness might have existed before then, but this is when it crystallized.
The first fandoms I started writing in were Buffy, The Vampire Slayer and Angel. First was role playing in the AOL chat rooms (remember those?), then onto to a RPG in a Yahoo! group. That led to solo stories on a shared web site with a group of fellow fanficcers, and then my LiveJournal. I’m still online friends with two people from this time.
Then The Lord of the Rings movies started coming out and that was my new focus. It also introduced me to Real Person fanfic, where we wrote about the actors instead of just characters.
I am currently doing Sherlock (BBC) fanfic with dalliances into a few other fandoms.
The easy thing about fanfic is also the hardest thing. Yes, you don’t have to go into detail about the universe you’re in, or in the descriptions of the people. But you have to remain true to the integrity of the universe. You have to keep to the rules, cause fans will call your shit out if you don’t.
One ugly aspect of fanfic that has evolved of late is the insistence of writers and fans to impose their personal desires upon the writers and actors of shows. Back when I started, *adjusts Dowager Duchess glasses* that was not the done thing. We would look at anyone who informed an actor of what we were doing with horror and derision.
We also tended to just stay away from pairings and categories that were not our cuppa, not feel the need to lash out with the vehemence you see today (unless you are talking slash, that will be covered later).
All in all, fanfic can be a good training ground, but there is only so much personal credit you can take for it.
If you are not outright persecuted for it.