The Leader was once an ordinary human (like most of Hulk's foes) but was exposed to similiar gamma-radiation as the Hulk and mutated into a grotesque green-skinned monster whose cranium was extremely swollen - probably due to his brain becoming so hyper-energized.
Ever since laying eyes on the Hulk the Leader became somewhat fixated on him, the two fought many battles and the Leader was prone to hiring notable opponents of the Hulk such as the Chameleon, Arsenal and the Rhino (who is actually more associated with Spider-Man) - in the comics he almost always remains a mad schemer and tends to fight the Hulk via minions, pawns or complex machines since he is physically weak.
In the cartoons however the Leader tended to create altered clones of the Hulk from samples of blood he had collected, in doing this he had an army at his disposal capable of tackling the Hulk - many of these clones were enhanced with cybernetics but were generally much weaker than Hulk or even She-Hulk - though the Ogress was fairly powerful, if lacking in intelligence.
The Incredible Hulk (2008 film)The Leader appeared in his pre-mutated form as Samuel Sterns in the 2008 film The Incredible Hulk, portrayed by Tim Blake Nelson. He is not seen until near the end of the movie and plays no part as a villain, however the origin is altered as it is exposure to Banner's blood that mutates the scientist rather than gamma-radiation - the creators of the movie have stated that if another movie is made they plan on having the Leader as a more prominent character, perhaps even the main antagonist.