DC Comics' premier superteam. They made their first appearance way back in 1960's The Brave and the Bold #28, right at the beginning of the Silver Age. The earliest adventures were written by Gardner Fox and pencilled by Mike Sekowsky. The Justice League are the guys who get to take on all the really dangerous threats to the Earth.
The original incarnation of the JLA included Superman, Batman, Wonder Woman, the Flash, Green Lantern, Aquaman, and the Martian Manhunter, but just about every superhero in the DCU has joined them at one time or another.
The best version of the comic book had its start in 1996, titled simply "JLA". It was the brainchild of Grant Morrison, though lots of other creators have left their stamp on the team, both for good and ill.
See also the JLA Issue Guide.