Fan fiction, or fanfic for short, is any original story using characters from a TV show or movie, or from another author's work, written by an author who doesn't own those characters. Fanfic is rarely done for money. It is a hobby, a labor of love for thousands of people around the world, from all walks of life.
One type of fanfic is the crossover, a fanfic in which the characters from more than one TV show or movie meet in the same story. Another category is the filk. A filk is a poem about a TV show or movie, set to the tune of a well-known song (which could very well be the theme song to yet another TV show or movie).
I write a lot of fan fiction, mostly about the television show Earth 2, since it is my favorite show of all time.
The first – and most irritating – question people ask me when I tell them I write fanfic is, "Well why don't you just write your own stories?" Please don't ever ask a fanfic author this question. It is so demeaning. Even so, I'll take the time here to give five answers to that question.
The first answer is the most obvious: they are my own stories!
What people really mean to ask is, "Why don't you use your own characters and settings?" This is a more accurate question, because I can assure you that although I am playing in someone else's sandbox, the sandcastles I build are original and wonderful, and they take a lot of work. There is an extra hardship, and an art form, to writing a fanfic story. Constructing a story which catches in print characters that were so alive on the screen, and creating events which slide neatly into the continuity of a television show's already-established events, is difficult. In these respects, it is actually more difficult than regular fiction. Writing fan fiction does not mean that I am creatively handicapped.
With this more accurate phrasing of the question, I can go ahead and give the remaining four answers.
The second answer is that I simply enjoy it. Writing fan fiction is what brings me happiness. It is my hobby, in the same way that other people have hobbies centered around collecting stamps or climbing mountains.
The third answer is that I love the television shows I write about, and I crave for more. If the television executives are not going to be smart enough to continue a show which shines with quality, then fanfic authors have to pick up the baton and carry on.
The fourth answer is that we fanfic authors write stories which would be otherwise impossible to enjoy. To emphasize this point, I call to your attention the series of Earth 2 / Doctor Who crossovers I have written. Even if both of these shows were still on the air, I would never have the pleasure of seeing their characters interact on the screen, yet I wish I could. So I do it myself in the form of literature, which is as close as I will ever get.
The final answer is that I do not write only fan fiction. I also write stories in which all the characters and all the settings are my own. However, they do not bring me as much joy, and I do not do it nearly as often.
Stories in which I own all the characters are commercial products for me. I have a chance of selling them someday. But an editor won't buy a story that has already been published, so you will not see any stories except fanfic posted on Spirit's Space.
I do not make any money off of my fan fiction. I do it for free, completely out of respect for the shows I love, and to celebrate them.