Muggles' Guide to Harry Potter/Magic/Protego

Overview
Protego is a spell used to guard its caster from incoming spells. It doesn't work against the killing curse. It is coined from the Latin verb 'protego', 'to protect' (lit. to cover over).

Extended Description
The effects of this charm seem to vary based on the desires of the spell caster.

In Harry Potter and the Order of the Phoenix, we see Harry using this spell against Professor Snape in his Occlumency lesson. Here, the effect is to reverse the spell being cast; Harry sees some of Snape's memories instead of the expected outcome, Snape seeing some of Harry's thoughts.

In the Battle at the Ministry late in that same book, Harry uses this spell twice. Both times, the spell being cast at him is simply interrupted.

In Harry Potter and the Deathly Hallows, Hermione casts this spell once, and Harry casts it three times. In these cases, the result of the spell is a "bubble" that is impermeable to spells and physical traversal (the bubble forming knocks Hermione back a few steps), though it is still possible to see and hear through it.

Also in the seventh book, several of the characters append this with other words. Professor Flitwick uses "Protego Horribilis" when he is preparing a shield for the school, and Hermione uses "Protego Totalum" when setting defensive spells around the campsite. The specific effects of the added words are not certain.