Created by the brilliant scientist Abraham Erskine, it grants whoever injects himself/herself with it amazing powers. However, the subject must then be subject to a special beam of energy known as the Vita Rays, or else he/she will suffer an horrible mutation.
Steve Rogers volunteered for a secret government project that revolved around injecting him with the Super Soldier Serum, which gave him superhuman strength and invulnerability, thus he became Captain America.
Emil Blonsky injected himself with the Super Soldier Serum to be a match for the Hulk. However, he tried to substitute the Vita Rays with the Hulk's gamma radiation-enhanced blood, which backfired when Emil mutated into the reptilian monstrosity known as the Abomination.
Johann Schmidt injected himself with the Super Soldier Serum. However, since he didn't expose himself to the Vita Rays, he mutated into the monstrous Red Skull.
The Super Soldier Serum was used as part of Project Deathlok.