proper usage of punctuation (hidden)

The style manuals devote many pages to this issue. I'll try to summarize in just a few words, but I will end up leaving out important exceptions and such.

Semicolons (;) have three uses. (1) To replace a period when joining two closely related independent clauses in a compound sentence. Normally, this is done with a comma + a conjunction (such as "and") but a semicolon indicates a closer relationship between the two parts. (2) To replace the commas in a serial list when the individual items in the list contain commas. This eliminates confusion as to where each item in the list starts and stops. (3) To replace the comma when "however" is used as the conjunction joining two independent clauses.

Colons (:) are used to replace periods when what follows is a list, or commas when what follows is a block quotation. A block quotation is a lengthy quote that is indented on both sides and set as a paragraph.

I hope this helps. 

These pages may also help:

http://www.grammarly.com/handbook/punctuation/colon/

http://www.grammarly.com/handbook/punctuation/semicolon/