Aquaman is one of DC Comics' oldest and most revered superheroes... well, maybe not by everybody. Actually, a lot of people tend to make fun of Aquaman, calling him the 'weakest Super Friend' and other hurtful remarks. Many of these people are later discovered dead, mysteriously drowned or the victims of shark or piranha attacks, near a large body of water...
Aquaman first appeared 'way back in the long-defunct More Fun Comics title in 1941, issue #73. This makes him one of DC's wartime heroes, though he's not often spoken of in the same breath as other DC golden age characters like the Sandman, Starman, Superman, or other members of the Justice Society. Aquaman was one of the backup strips in More Fun for several years before moving to a long but sporadic run in Adventure Comics. He would get his own title in 1962.
Aquaman was reputedly the child of a surface-world father and a mother who came from the undersea kingdom of Atlantis (an origin story a little too similar to that of Marvel's Sub-Mariner, who debuted two years earlier). He could live and breathe underwater, of course, and his strength increased dramatically when immersed in brine; he could also communicate with, and partially control, underwater creatures. His powers and circumstances made him an ideal superhero to fight German and Japanese naval targets in World War II adventures, and naturally he was seen dispatching German U-boats with the typical military glee of the time. After the war he would take on the usual pirates, corrupt shipping magnates, and several ocean-themed super-villains. During the late 1940's and early 50's, Aquaman was one of the few super-heroes (including Superman, Batman, and Wonder Woman) who could be found on the newsstands.
The Silver Age Aquaman series was the most visible and important run of the character's adventures; here his origin would be firmly worked out and various supporting characters introduced. He would become king of Poseidonis, the new home of the dispersed Atlanteans; he would marry an other-dimensional undersea beauty named Mera; he would father a son, as well as adopt a youth who would become the superhero called Aqualad. Aquaman would become a founding member of the Justice League and later (on television) one of the original Super Friends. Starting in the 1980's, continuing with a comics-wide trend that shook up the superhero world and introduced dark aspects into many characters' lives, Aquaman would suffer various defeats and indignities to finally become a grim-&-gritty, one-handed, bearded avenger.
Aquaman is today a popular DC hero, mainly due to this modern darker treatment of the character in the comics, as well as a short-lived appearance on the Justice League Unlimited cartoon. The WB network is making plans for an Aquaman TV show, probably along the lines of the Superboy vehicle Smallville, in which a version of the character (more or less) made an appearance. Not to dash any classic Aquafans' hopes, but it will probably just end up being a guy in a wetsuit who can swim pretty good.