Colon:

Skip two spaces after a colon.

A colon is used:

To introduce a list and emphasize the elements of this list. 
The sentence introducing the list is complete.
Example:
A computer contains four drivers: a hard disk, a CD drive, 
a DVD drive, and a Soft-drive. 

Even if the sentence is not complete, a colon with a tabular form can be used.
Example:
A computer contains:
A Hard disk,  
A CD drive, and 
A Soft-drive. 

 
To introduce a list whish emphasizes the first sentence
Example:
She has an advice: Learn and be careful.

List is introduced by words such as: these, as follows, and the following.

 Between two clauses when the second emphasize the first.
Example:
She failed the exam: A lot of questions were very difficult.
  
Before a formula which reaffirms the preceding sentence
Example:
If you know the length “L” of a pendulum, you can know its period “T”:
	T=2PIsqrt[L/g] 

To introduce a long quoted speech (a comma is usually sufficient)

In formal business letters: Dear: Doctor: 

To express a ratio: 2:1 

 Between hours and minutes in time: 11:30 P.M.

Don’t use a colon:
When the sentence introducing a list is not complete, but 
completed by the list itself.
Example:
A clause contains 1) A subject, 2) A verb.