The Hulk is one of the best known superheroes in comic book history. Despite his heroic status among the rest of the Marvel universe's superpowered community, he is more often than not viewed as a monster by society.
Doctor Robert Bruce Banner was once a meek, socially withdrawn scientist and the foremost authority on the study of gamma radiation. He had suffered a traumatic childhood, during which his father would physically abuse his mother, which had a profound psychological impact on him. He developed a complex of fear towards anger, leading to him repressing his anger later in life.
Banner first transformed into the Hulk during a field test of his new gamma bomb. As the test began, a drifting youth called Rick Jones wandered on to the testing area and Banner ran out to save him. Although he saved the boy's life, Bruce was saturated with gamma rays when the bomb detonated, but instead of being killed, the radiation radically altered his body, transforming him into the green-skinned goliath known universally as the Hulk.
The gamma blast also had a significant effect on Banner's mind. While Bruce was normally quiet and reserved, the angry feelings he had long surpressed were awakened with his transformation, manifesting as the Hulk's own personality. Prone to bouts of extreme rage, the Hulk would come to be regarded as a savage, mindless brute by society at large. This has led to Banner being hunted by the US military and numerous villains and even heroes.
Bruce Banner had always been emotionally repressed and since his transformation into the Hulk has been forced to train himself in various methods of meditation and other self-control exercises in order to keep his Hulk personality in check. Typically, Banner comes across as nervous and shy, although he is also a brilliant scientist and enjoys having new challenges presented to him, especially if it helps take his mind off worrying about the Hulk.
For a long time, Hulk would behave in a savage, brutish manner, often speaking in the third person. In recent years, however, he has been portrayed as moderately intelligent, although he can still be rather bad-tempered and is not as smart as his Banner personality. Hulk normally disassociates himself from his "puny" Banner persona, though there have been occasions where the two personalities have relied on each other to overcome a particular threat. Although prone to anger, Hulk does care about others and regularly puts himself in harm's way to protect innocents.
As Hulk often says himself, "Hulk strongest one there is." And that's not an overstatement. When Banner becomes the Hulk, he gains phenomenal physical strength and endurance. The Hulk's power is greater than any known being to have walked the Earth and seems to have no limit, fluctuating depending on his emotional state. His strength increases with his rage, making him all the more dangerous the madder he gets.
Hulk has displayed the ability to lift over 100 tons, lifting objects such as cars, airplanes and giant robots with minimal effort. His punches can level buildings, he can jump tremendous heights and distances (including across continents and into orbit) and even clapping his hands together can create violent shockwaves.
For all intents and purposes, Hulk is indestructible. His superhuman endurance allows him to withstand bullets, heavy impacts, even nuclear explosions. Any damage he does sustain rapidly heals and his body has even displayed the capability of generating new tissues that allow him to survive in inhospitable environments such as underwater and even the vacuum of space.
Hulk has no conventional weaknesses and not many unconventional ones. History has shown him to be vulnerable to certain outlandish technological devices specifically designed to harm him; there have been occasions where others have attempted to siphon the gamma energy from Hulk's body, which has proven to be at least somewhat effective.
Hulk is most vulnerable when in his Banner form, who is only as strong as an ordinary human. However, the Hulk has proven that he is aware of his surroundings even when Banner is in control and Banner's transformations can be surprisingly swift sometimes. Banner has on at least one occasion attempted suicide in order to rid himself of the Hulk, but has involuntarily transformed and prevented his death.