It's a webcomic.
It's also one of the most spectacular pop-culture phenomena of the past decade, beloved by millions of readers: a unique and massive internet-based narrative work consisting of comics, chat logs, gifs, games, animation, and music.
It's a tale about a boy and his friends and a game they play together.
On his 13th birthday, John Egbert starts playing a mysterious videogame called Sburb. Unfortunately, this triggers the apocalypse. Fortunately, he and his friends can make things right—if they can beat the game. They’ll need a lot of teamwork, a little luck, and some inspired shenanigans along the way to make it through this mind-bending, genre-defying adventure.
A visual key to the Acts and Parts of Homestuck, and a chronological log of every page.
Confused? Disoriented? Yes! Check out the Homestuck Wiki on FANDOM.
Another great guide is The Acts & Pages of Homestuck.