Semicolons ( ; ) are used to join closely related independent clauses. In other words, they join two or more closely related sentences together. Example:
"I saw Uncle Bill's deals today; sadly, I don't have any money." Basically, if you can substitute a period for the semicolon, you're using it correctly. The biggest misuse of the semicolon is to join a sentence fragment to an independent clause. Example:
"I watched the Perseids last night; then went to bed."
In this case, what follows the semicolon isn't a complete sentence, so it won't work.
The only other place semicolons are used is to separate items in a list where one or more of the items already contains a comma.
Example:
"I invited Bob; Maria, who is a lawyer; Linda; Terry, a nurse; and Roger to the party."
Even if only one of the items in the list contains a comma, all of them are separated by semicolons.
Colons ( : ) on the other hand, are used for a variety of things, including time and separating the chapter and verse in bible citations. They are also used to set something off from the rest of the sentence while at the same time drawing the reader's attention to it. Things that can follow a colon include a direct quotation, a list, or an appositive. An appositive, by the way, is a word or phrase that renames or describes a noun.
Examples:
Appositive:
Caroline finally purchased her dream car: a Mazda. (Mazda in this case it the appositive because it renames or gives additional info about "car")
Direct quote:
On the issue of convex edges, Josh Feltman said the following: "Strop 'em if you've got 'em."
List:
To put a Tru-Oil finish on a khukuri handle, you'll need the following items: Tru-oil, varying grits of wet/dry sandpaper, furniture wax, and lots of patience.
I'll shut up now. Lemme know if this isn't clear.
--Josh