Homestuck: A Newbies' Guide
It's honestly hard to say what Homestuck is.
But here goes: Homestuck is a comic-type story told in mixed media (text, images, flash games, animations) which is initially about four teenagers who play a game that creates and destroys universes. And then about twelve trolls who play a different but linked session of the same game. And then about the genetically linked children/parents of the first four teenagers.
By page count it's one of the longest stories in the English language, running at over ten thousand pages (compare to the mere 1000 or so in all three parts of The Lord of the Rings, though still somewhere short of the 12000 or so in the full Wheel of Time series). It's also notable for having an obsessive fandom who are often looked upon with confusion (unsurprisingly, as fans of a work that draws from sources as diverse as 1980s adventure gaming, 1910s light music, the zodiac, and lucky charms cereal).
And this is the place to discuss it! (And its smaller cousins such as the excellent Problem Sleuth).