Homestuck is a webcomic. It's the longest one that I've ever seen. The author has a wicked sense of humor and dialog writing that is up there with the likes of Kevin Smith and Joss Whedon. He also uses more than the normal webcomic panels, incorporating flash animation, gif animation, and music. The comic follows a format that mimics old text based adventure games, with each page being a command and the panel being the results of that command. In previous adventures the commands were submitted from the audience. In homestuck they're made by the author, but what this lacks in gimmic is made up by the fact that story is tighter and more impressive with less silly commands and sidetracking.

The story itself is about a boy named john who plays a video game with his friends that changes the world. I really can't say too much more without spoiling things a little. The story starts off a bit slow, but if you enjoy it right away you'll be set the whole way through. A lot of people don't like it at first but find that it gets a lot better and more impressive as you go on. The first two acts are the biggest barrier to this comic's success, but they're still pretty good in their own way.

The fandom has a few nutjobs like any fandom does, but this comic inspires a sort of rabid fanaticism, so our fans are a bit more vocal and most. Homestuck cosplay groups at conventions have been known to rival the Naruto or Final fantasy groups.

If you're still interested in reading it, give me a message sometimes, I love getting new readers into it. You can find the comic here: http://www.mspaintadventures.com , but you may find it easier to get into Problem sleuth, the adventure that was finished just before homestuck started. Homestuck is currently almost four times longer than PS, but PS is a complete work, and a great introduction to the writing style, the running gags, and the general sense of scale that homestuck follows.

Source for all the above: I'm a mod of /r/homestuck.