Fanart is without a doubt the most popular fanmade content. It's drawings, comics or cartoons of the show and it's characters, sometimes just capturing memorable moments and other times they just build on on the story or picture the characters in 'alternative universe' situations. The most famous example is a comic made by Maryne, mostly known by the fandom as 'THE COMIC'. It can be read here.
Fanfiction are fanmade stories about the show and it's characters. They can be close to the story of the show but they can also deviate alot from it's story, settings, and even characters. The most famous example is Fool's Gold, which you can read here, on Archive of Our Own, the most known fanfiction site.
Crackposts are screencaps of the show with different text added, which almost always ensues to a hilarious result. Carmillacrackposts is the biggest 'supplier' of these type of posts.