Lumos

Saying 'Lumos' will allow the caster of the spell to make a beam of light appear from their wand.

Saying 'Nox' would put it out.

Expecto Patronum

Expectro Patronum is the incantation of the Patronus Charm. The spell can only be cast when thinking of 'something happy', with your innermost optimistic emotions, as it is a 'positive force'. It is used against Dementors and takes form of an animal.

Expelliarmus

The Disarming Charm is a spell which lets the spell caster disarm his/her opponent of their weapons, especially their wands.

Avada Kedavra

Of the three Unforgivable Curses, this is the Killing Curse. A bright green flash of light and a rushing sound occur when the spell is cast. It immediately kills those who receive the curse, though no bodily harm happens to them. There is no counter-curse that will protect you against the curse.

Only Harry Potter has survived after a direct hit – two, actually.

Crucio

The Cruciatus Curse, one of the Unforgivable Curses, tortures those who receive the curse, causing incredible pain to them, as if 'white hot knives' drive through your very skin.

Imperio

The Imperius Curse lets the caster manipulate another person's actions (after casting it on them). Only those with strong will are able to resist it. It is one of the three Unforgivable Curses.

Stupefy

The Stunning Spell allows the caster to stun his/her victims. Stupefy is the incantation of the Stunning Spell.

Accio

Accio is the incantation of the Summoning Spell. It allows the caster to summon objects from a distance.

Obliviate

The Memory Charm, of which you would say 'Obliviate', is used to wipe away one's memories of an event.

Wingardium Leviosa

Wingardium Leviosa, the Levitation Charm, lets the caster levitate objects.

Aguamenti

Aguamenti makes water appear out of the caster's wand.

Confringo

This spell causes things to blow up into flames when cast upon them.

Protego

The Shield Charm can protect the caster from minor curses, jinxes, and hexes so that it rebounds upon those who cast the curse, jinx, or hex in the first place.

Morsmordre

Morsmordre is the incantation to conjure the Dark Mark.