The Justice League is a team consisting of four (originally five) superheroes.
Before Arkham Knight Incident
At some point, a group of superheroes bound together to form a team known as the Justice League. They formed a base of operations containing statues of each member, and a large table used for group meetings. Batman, Flash, Green Lantern, Superman, and Wonder Woman were the members of the Justice League.
After The Knightfall Protocol
Following the supposed death of Batman, the rest of the League continued their operations. Some unknown amount of time later, they became subject to a kill-order by Amanda Waller: the leader of the Suicide Squad.