Here's some spells and what they do that are in the Harry Potter books. If I missed one, e-mail me and tell me it, and what book it's in, and the page number, and what it does. ;)
Accio - causes and object to fly to the person who casts the spell. But the person must have a general idea of where the object is and what it is.

Alohomora - causes a locked door to open.

Aparecium - makes invisible ink appear.

Avada Kedavra - kills instantly in a flash of green light.

Avis - conjures a small flock of birds.

Crucio - causes almost intolerable pain and in some cases has driven the person to insanity.

Deletrius - erases the ghost images of spells created by the Priori Incantatem spell. Maybe can remove other spells effects, too?

Densaugeo
- causes a person's teeth to grow grotesquely.

Diffindo
- cuts something open.

Dissendium - opens the secret door in the statue of the humped witch.

Engorgio - causes something to swell in size.

Ennervate - revives someone who's been hit by a Stunner.

Expecto Patronum - conjures a Patronus, which is a silvery wispy usually in the shape of an animal that is made from the happy thought of the caster. It drivers off Dementors.

Expelliarmus
- makes the opponent's wand fly out of their hand.

Ferula - conjures a wooden rod.

Finite Incantatum - stops the current spell effects.

"Point Me" aka, The Four-Point Spell - when laying your wand in the palm of your hand and say that, the wand will point North.

Furnunculus - causes boils to break out on the victim.

Impedimenta - stops or slows an object down.

Impervius - makes something waterproof.

Imperio - makes the caster be in complete control of the victim and makes them do anything the caster wants. It can be broken down by the victim's strong mind, however.

Incendio - starts a fire.

Locomotor Mortis - makes the victim's legs lock up so they can't move.

Lumos - causes light to emit from the end of the caster's wand.

Lumos Solarum
- causes a really bright blast of light, like the sun.

Mobiliarbus - moves a tree.

Mobilicorpus
- moves a body.

Morsmordre - causes a glowing green skull with a snake coming out of it's mouth in the sky. Used by Death Eaters and is the sign of Lord Voldemort.

Nox - turns off the light from a Lumos spell.

Obliviate - modifies/erases part of a person's memory.

Oculus Reparo - repairs some eyeglasses.

Orchidious - causes flowers to spring from the end of the caster's wand.

"Pack!" - causes items to pack themselves in a trunk.

Petrificus Totalus - turns the body of the victim rigid.

Prior Incantato
- causes a wand to emit a ghost image of the last spell it cast.

Quietus - reverses the Sonorus charm and returns the person's voice back to normal.

Reducio - causes something to return to it's normal size. (If it was enlarged with Engorgio."

Relashio - causes the wand to emit fiery sparks, or jets of boiling water if you are underwater.

Reparo - fixes a damaged object.

Rictusempra - causes a person to laugh uncontrollably.

Riddikulus - defense against a Boggart, and makes it take on a silly appearance.

Serpensortia - causes a serpent to burst from the end of the caster's wand.

Sonorus - makes the caster's voice loud and carry over long distances.

Stupefy - makes the victim unconscious.

Tarantallegra
- causes the victim's legs to do a crazy dance.

Waddiwasi - shoots a wad of gum out of a keyhole.

Wingardium Leviosa - causes an object to levitate.
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