The rules below are based on the Chicago Manual of Style (CMOS).  Please note that there are slight variations and preferences when using other style guides.

Main Uses of the Colon

The main use of the colon is to draw the reader’s attention to a list, appositive, or quote following an independent clause. The use of the colon is similar to saying as follows or the following and should be used to emphasize that what follows is directly related to, illustrates, or amplifies what precedes the colon.

List

Appositive

Quotation

Other Conventional Uses

Other common uses of the colon are following a salutation in a formal letter (To Whom It May Concern:), time (7:45p.m.), ratios (2:1), chapter and verse citations (Matthew 22:37), and in certain cases for bibliographic citations (Title:subtitle).