Robert Bruce Banner was a scientist, working at Culver University, when he was recruited by the U.S. Army to be part of their attempt to re-create the super-soldier serum, that created Captain America. Banner became sure, that his serum would make humans immune to radiation. So sure, in fact, that he had himself injected with the serum and exposed to a high amount of Gamma Radiation. The combination of the two, caused Banner to transform into the Hulk for the first time, and hospitalized his former Harvard co-ed/girlfriend, Dr. Elizabeth "Betty" Ross. While Betty was at the hospital, Banner learned the truth about what he had been working on. He had believed that the serum was intended to radiation resistance, but found out that the Army wanted to duplicate his DNA and make it into a weapon. Banner decided to escape and tried to keep himself hidden, searching for a cure.
Powers
Metamorphosis: When Bruce gets angry or excited enough to increase his heart rate to high levels, he transforms into a powerfully muscled and green version of himself, generally referred to as the Hulk. In his earliest transformations, Banner appeared as a savage with a regressed child-like state, and with no real ability to distinguish friend from foe. Initially, he was unable to control the transformations, but through meditation and breathing techniques, has managed to attain some level of control. Banner's consciousness is buried within the Hulk's, and can influence the Hulk's behavior only to a very limited extent such as when he changed into the Hulk to fight the Abomination, where he managed to possess some human traits when asked by Betty Ross not to kill his enemy.
After over a year of training to control his transformations, Banner learnt that if he always remains angry in a controlled state, then he can easily change into the Hulk at will, into a smarter and less enraged state where he is able to show affection and demonstrates a capacity to work collaboratively with others. However, if Bruce is sent into a state of stress, then he can still lose control and transform against his will despite trying to control it. This Hulk seems to revert back to a savage enraged state, where he would attack anything or anyone ,be it friend or foe.
Superhuman Strength: In the form of the Hulk, Bruce possesses immense physical strength, which increases in proportion with his anger. As the Hulk he was able to rip steel as it was made of paper, and was able to throw heavy objects with tremendous force "like a softball". While maybe not as powerful as he is in the comics he is more powerful than anything within the world of the movie. He is also capable of leaping great distances. By clapping his hands he was able to make a shockwave that snuffed out a fire. His strength is only rivalled by the Abomination or Thor. In times of stress the Hulk's adrenaline level escalates, causing a corresponding escalation in strength. This is not accompanied by an additional gain in mass, but does appear to promote increased levels of energy efficiency. To date the Hulk has never apparently been provoked into demonstrating a maximum output of strength, hence his upper limit remains a mystery. However, whatever the limit, it is well in excess of 100 tons. This is also displayed when seeing Betty in danger in the damaged helicopter, he outmuscled the Abomination to free himself. By raising his fists and arms and striking the ground, he can create tremors of incredible force that stun the Abominaiton.
Superhuman Speed: Regardless of his size, the Hulk's superhumanly strong legs allow him to run at speeds that are far beyond the natural physical limits of even the finest human athlete. Nevertheless, this does not grant him any enhanced reflexes or agility as smaller foes exploit his size and use it to their advantage.
Superhuman Durability: The Hulk is able to withstand substantial punishment; even high caliber bullets bounce off of him and the fire causes much less damage compared to a human. His bone, skin and muscles are overly dense, hard and tough and he possesses a high degree of resistance to injury, pain, and disease. Only beings with tremendous force or extremely powerful weapons can harm him. The Hulk also withstood multiple energy blasts from Chitauri weaponry, maximized cold without freezing in the Arctic Circle, and great falls from heights of over six miles.
Superhuman Endurance: Substantially greater endurance than a normal human. The Hulk's body counteracts fatigue poisons that build up in his muscles during physical activity, allowing the Hulk to run and fight for a long time. While fighting others in an enraged state, he can maintain peak output for hours on end and still continue to become even stronger as his anger escalates.
Superhuman Healing Factor: The Hulk displays an incredibly fast rate of regeneration. This grants immunity to any illness and longevity (for Hulk, not Bruce). The Hulk heals rapidly from injuries at a much greater rate than a normal human. Grievous bodily harm has also proven to be a trigger for Banner's transformations, and he has recovered from otherwise fatal injuries sustained in his normal form. Banner said he once tried to put an end to his miserable existence by putting a bullet through his mouth, but the "other guy" spit it out.
Immunity to Diseases and Viruses: Hulk has a enhanced immune system ensuring his immunity to some unrevealed Earth-based diseases and viruses; Dr. Samuel Sterns described that a synthesized Banner's blood sample would make humans impervious to disease.
Abilities
Bruce Banner is considered one of the greatest minds on Earth, and one of the foremost geniuses of his generation. He has developed expertise in the fields of biochemistry, engineering, and physiology, and holds a Ph.D. in nuclear physics. He possesses "a mind so brilliant it cannot be measured on any known intelligence test." He is a highly rated expert in gamma radiation and according to Tony Stark, his work on anti-electron collisions is unparalleled. He learned Aikido and meditative breathing basic techniques, while in Brazil, with a martial arts expert.
As the Hulk, while having no formal training, he is a formidable hand to hand combatant simply due to his great physical attributes. He typically uses street fighting and brawling techniques that make full use of his strength.
Weaknesses
As the Hulk, Dr Banner is hindered by a vastly reduced intelligence. His scientific mind regresses to a very primitive state, barely capable of deductive reasoning. Coupled with his extreme rage and physical power, this can cause havoc as he vents his rage on the world around him. As the rage subsides however, his emotional attachments to particular people, or Banner's concern for innocents, have allowed him to regain some level of control and turn his prodigious strength towards productive outlets.