R+L=J is one of the biggest and oldest Game of Thrones / ASOIAF fan theories. Who is Jon Snow's mother? Is Jon secretly a Targaryen?
This video contains major spoilers for Book 1 / Season 1, and some minor spoilers/references for Book 2 and Seasons 2-4. Basically, if you've read Book 1 or seen Season 1, you can watch this video without having anything significant spoiled.