A stimpak is an autoinjector loaded with a variety of healing medications and stimulants. By injecting the stimpak, one drastically increase one's own recuperative functions and gains lost hit points almost instantly.
Stimpaks immediately restore between 12 and 20 hit points, making them the go-to healing chem for most of the game. Super stimpaks are a more effective version, especially in combat (though this point is moot, as opening the inventory gives unlimited time to use chems).
Vault 13: Several received at the start. More available from the level 3 store house or the EML technician.
Trying to use a stimpak from the active item slot if the player has not enough action points will still use them up, without healing.
↑Vault Dweller's Survival Guide p.5-19: "A stimpak (short for Stimulation Delivery Package) contains many healing chems. A soup of healing medication, if you will. By injecting the Stimpak, you drastically increase your own recuperative functions and restore lost hit points almost instantly."
↑Fallout Official Survival Guide p.124: "Stimpak: The stimpak is an autoinjector loaded with a variety of healing medications and stimulants. Injecting yourself or another character will increase your natural healing ability and give you an immediate Hit Point boost."