The Stunning Spell (incantation Stupefy) is a spell that knocks the victim unconscious. It can also apparently be used to halt moving objects (Professor Umbridge uses the spell to unsuccessfully stop the Weasleys' dragon fireworks). It is often used in dueling to disable opponents. Ministry of Magic personnel also use it to stop criminals.
The Stunning spell is relatively easy to stop and can be deflected by the Protego charm. It is next to useless on large magical creatures unless several people fire the spell at the same time. Species known to be resistant to the spell include dragons, trolls, giants and half-giants (Hagrid is stunned multiple times with no effect). It is also possible to bewitch magical objects to be resistant to the spell (Fred and George Weasley bewitched their fireworks to explode when hit by stunning spells) Stupefy is shown to have a two color variety. In the books, it is described as a "jet of red light." But in the Harry Potter and the Order of the Phoenix movie, it's shown as a blast of bluish-white that just knocks people off their feet.
The countercurse is Ennervate. The spell will eventually wear off, even if the countercurse is not used. It is first used in the beginning of Harry Potter and the Goblet of Fire in the forest after the Quidditch World Cup. .
"Stupefy" comes from the Latin "stupere", which means "to be struck senseless". adebola said quickly out of breath