A Hero is a man of distinguished courage or ability, admired for his brave deeds and noble qualities.
Spider-Man: The bite of an irradiated spider granted high-school student Peter Parker incredible powers. When a burglar killed his Uncle Ben, Peter vowed to use those abilities to protect his fellow man, driven by his uncle's words: With great power comes great responsibility!
Captain America: Steve Rogers was unable to enlist in the U.S. due to his poor constitution. After rigorous physical and combat training he was selected as the first test subject in a new program. The process successfully took him from his frail state to the maximum of human efficiency, giving birth to Captain America.
Iron Man: Tony Stark was attacked and taken hostage while field testing one of his military weapons resuilting in a piece of shrapnel being lodged near his heart. He developed a battle suit and used it to free himself. Reluctant at first to accept his new role as a hero, he went on to form the world's first superhero team - the Avengers!
Thor: Odin, ruler of Asgard, and Gaea created a child of both Asgard and Midgard (Earth). Thor was taken to Asgard to be raised by Odin's wife Frigga. On Thor's eighth birthday, Odin had the magical hammer Mjolnir created, to be presented to Thor once he proved to be a worthy warrior. Eight years later, he succeded and Odin presented Mjolnir to Thor, declaring him the greatest warrior in Asgard.