Accio - Accio is a summoning charm, used whenever one wants to summon an object. In Harry Potter and the Goblet of Fire (4th movie), Harry used this charm to summon for his broom in the first task of the Triwizard Tournament. "Accio Firebolt."
Wingardium Leviosa - Wingardium Leviosa is a levitating charm, used to fly/levitate an object. We are first introduced to this spell in Harry Potter and the Sorcerer's Stone (1st movie). In Charms class with Professor Flitwick, Hermione casted this spell to fly a feather.
Obliviate - Obliviate is memory charm, used to erase one's memory. The spell was first introduced in the second book, Harry Potter and the Chamber of Secrets, used by Gilderoy Lockhart. The use of this charm appeared more frequently in the latest movie, Harry Potter and the Deaathly Hallows Pt. 1, by Hermione. Hermione casted this spell on her parents to make them forget about having a daughter, and to make them believe that their life goal was to spend the rest of their lives in Australia.
Alohomora - Alohomora is a spell used on locked doors, so as to unlock it. Appeared first in Harry Potter and the Sorcerer's Stone.
Aguamenti - Aguamentiis a charm that produces clear, pure water out from the caster's wand. It was first demonstrated in Harry Potter and the Half-Blood Prince (6th movie), by Harry.
Expecto Patronum - is a Patronus Charm used as a shield/protector against Dementor. In Harry Potter and the Prisoner of Azkaban (3rd movie), Harry was taught this charm by Remus Lupin, who was Harry's Defence Against the Dark Arts teacher that year.
Lumos and Nox - Lumos is used to produce a beam of light which shines from the wand's tip. Nox is the counter spell of Lumos, which will extinguish the light.
Homenum Revelio - this is a spell used to reveal any existence of human near the caster. In the Deathly Hallows, Hermione casted this spell upon arriving at Grimmauld's Place, after the attack of Death Eaters, after the wedding.
Expelliarmus - Expelliarmus is a Disarming Charm that appeared quite so frequently throughout all the movies. This spell is used to disarm an opponent. Casting it on an opponent will cause the opponent's wand to fly out of reach.
Protego Totalum - Protego Totalum is a Protective Charm. It is one of the charms used by Harry and Hermione in Deathly Hallows to protect their camp site from unwanted visitors.
Reparo - Reparo is a charm used to repair broken or damaged objects. Appeared many times throughout the series.
Finite Incantatem - This spell is used to stop most spells. It negates the effects of many spells. In Deathly Hallows, Hermione suggested Ron to use this spell in order to stop the rain in Yaxley's office, at the Ministry of Magic.
The Avada Kedavra
This is the Killing Curse. It is one of the 3 Unforgivable Curses, which means the use of this spell may cause the caster a life sentence in the Prison of Azkaban. There are no counter curses for the Killing Curse - it is unblockable. Although it does require the caster to be a powerful wizard in order to be able to cast an effective Killing Curse.
The only person (other than Voldemort) known to have survived the Killing Curse was Harry, whom Voldemort tried to kill when he was a baby. But the Killing Curse casted by Voldemort backfired and hit Voldemort instead. Voldemort did not die, though, because he has made Hocruxes. The survival of Harry from this killing curse is what made Harry famous in the wizarding world as he is the only person to have survived and was called as "the boy who lived."