look up any word, like guncle:
 
1.
the sign (:) used to mark a major division in a sentence, to indicate that what follows is an elaboration, summation, implication, etc., of what precedes; or to separate groups of numbers referring to different things, as hours from minutes in 5:30; or the members of a ratio or proportion, as in 1 : 2 = 3 : 6. (From Dictionary.Com)

You might think of a colon mark as saying "Watch what's next" or "Read on for more information."
Here is a fact about the colon mark: A colon is different from the semicolon.

or

The following colors are available:

- Red
- Blue
- Green
by MadamDeb November 15, 2010