A colon introduces a list or explanation, as in “You will need: eggs, flour, and milk.”
The colon is a punctuation mark made of two vertically aligned dots. In modern English, it commonly signals that what follows will explain, define, illustrate, or enumerate what came before. The words before it should generally form a complete clause: “She brought three ingredients: eggs, flour, and milk.”
Colons have several other jobs. They can introduce a quotation, separate hours and minutes in a time such as 9:30, divide chapter and verse in Bible references, and show ratios such as 2:1. In formal American-letter conventions, a colon may follow the greeting “Dear Editor:”.
A common mix-up is the semicolon, which normally links closely related independent clauses or separates complex list items. A dash can also introduce an explanation, but it creates a different, more abrupt emphasis. The colon is the standard mark when the second part is directly promised by the first.