Colons do several things. They're a highly useful little marks of punctuation.
Colons indicate that you're about to elaborate.
Rachel's plan was simple: camp out for as long as it took.
Colons introduce lists.
She brought the following items: a sleeping bag, a homemade sign, Tropical Twist gum, a Team Jetpack t-shirt, and her friend Amy.
Be careful: don't use colons to introduce lists that have already been introduced by verbs or prepositions.
NO: Side effects included: cold hands, a pounding headache, and a sore butt.
YES: Side effects included cold hands, a pounding headache, and a sore butt.
Colons signal definitions.
Twihard: A Twilight fan willing to lose hours of sleep and risk a sore butt in the pursuit of New Moon tickets.
Colons introduce commands.
You, in the Team Edward t-shirt: tell me exactly what your problem is. Do you like possessive control freaks?
Hey, you: Got a question for me? Email firstname.lastname@example.org.