No, but seriously, it kind of is. Because of that, I hope you forgive me the sheer scroll-iness of this post; I've got a lot of ground to cover
Fanfiction (or fan fiction; you can write it either way) is any original writing based on an existing work, such as a television show, a movie, or a book. Usually, fanfiction involves characters and places from this work, but it can also just take place in the same world. How closely related it is to the original is up to the author.
This probably sounds like copyright infringment, and to a point, it is. With rare exceptions, fanfiction can't be published for financial gain, and there is even a (very small) group of authors who ask fans to not publicly share fanfiction online, even if it's for free.
As disappointing as that might seem, the point of fanfiction really has nothing to do with money. The primary reason to write fanfiction is to more fully enjoy and connect with the original. Side benefits include connecting with other fans, indulging in some wish fulfillment, and even improving your writing. To anyone who has read the notoriously catastrophic/beautifully troll-tastic fanfic "My Immortal," that probably seems like a lie, but I swear to you, fanfiction has a lot to offer.
So, what do I mean by "wish fulfillment?" Fanfiction just takes a story and retells it, right? Not so, friends. Not so. There are two major camps of fanfiction: there's "canon," which applies to stories that that use only information given in the original, and then there's "fanon," which can twist or even completely change ideas from the original work. Example: a canon Harry Potter fanfic could include Ron and Hermione spending time with their kids at Christmas, while a fanon Harry Potter fanfic could be about Neville and Hermione's wedding.
A personal example: I just burned through all seven seasons of Buffy the Vampire Slayer. Aside from the soul-crushing sadness of finishing the series, it also gave me a new favorite fictional couple: Willow and Xander. Though the two of them finally had a very brief fling, they never ended up together, which always disappointed me. Right now, I'm working on fanfics that have the two of them together so I can enjoy the stories it would create while still being able to enjoy the show for what it is. It gives me a release valve for that disappointment; instead of having my enjoyment of the show diminished because I didn't get to see the relationship I wanted, I can basically have my cake (the show) and eat it too (the fanfic).
"Very nice - albeit slightly pathetic - example, Allison," you say. "But how could it help improve my writing?"
Whether you realize it or not, when you're writing fanfiction, you're thinking very carefully about details, consistency, and voice. Really good fanfics, for the most part, are the ones that are true to the heart and soul of the original, regardless of whether or not other changes have been made. I'll go back to my Buffy example. Willow and Xander are both characters with very clearly defined personalities and distinct ways of speaking. If you really pay attention, most major characters do. That means that, to write them well, I have to make them act like themselves. I have to think very carefully about what they would really say or do in different situations. Imagine what it would be like if they broke character in the show. It would be really awkward and distracting, right? It's the same in fanfiction.
Unless you're doing an extreme deviation from the original (like something set in an alternate universe), you also have to pay attention to detail. Back to Buffy: let's say I'm writing a story that takes place during season 3. This means it will probably take place in Sunnydale, CA, so I have to think about setting. They're probably not going to be getting out of school for a snow day there. I also need to think about what kinds of things they're going to be doing. Most likely, they'll be hanging out at their homes, their school, and the local club (The Bronze), and they'll probably be spending time with Buffy, Giles, Cordelia, and Oz, unless I choose to deliberately eliminate those characters. I'll probably at least need to make nods to different villains (vampire or otherwise) and I may choose to talk about details from past episodes, like character nicknames or experiences they had together. This might require research; if I can't remember how Willow could tell between the two different Xanders in a later episode, I will have to look it up. (Answer: he did the Snoopy dance from The Charlie Brown Christmas Special.)
This is all stuff that you can apply to a lot of different types of writing, even things like essays. Learning to write in a way that mimics the voices of certain characters will help you work on academic voice in papers by thinking about how professionals and academics speak versus how students talk to their friends. Learning how to pay attention to details and do research to remember them will help you put a lot of detail and thought into your analyses and arguments. And, of course, doing any types of writing will strengthen grammar, spelling, and other writing skills.
OVERALL: Fanfiction is a really fun, really interesting form of writing that can offer unique help and practice in some really crucial writing skills. If you want to share your work online, Fanfiction.Net is one of the best and easiest places to do so. I hope fanfiction will help you find inspiration and writing passion the same way it has helped millions of others over the years.
