Justice League Heroes Unite Collection
The Justice League is a cast of established superhero characters from DC Comics' portfolio that include: Green Lantern, Batman, Superman, Martian Manhunter, the Flash, Wonder Woman, and Aquaman. On the Heroes United t shirt, six of the seven original members are seen. In the Justice League Heroes Unite the message is clear they will unite as a team that is dedicated to fighting crime and injustice, and act as Earth's first line of defense from terrestrial, extra-terrestrial and inter-dimensional threats.
The team first appeared in The Brave and the Bold #28 (March 1960). The tJustice League was conceived by writer Gardner Fox as a revival of the Justice Society of America, a similar team from DC Comics from the 1940s.
To read about the Justice League as presented in DC Comics go to this DC Comics post.
Or this wikipedia page titled: Justice League
Or read about the Justice League in this article on GoMart.com: History and Evolution of JLA (Justice League of America)