Colons are often confused with semicolons, but have
completely different uses.  Colons are used in the
following ways:

* Lists

Colons introduce a list.
This is what I need from the market:  potatoes, eggs, and
bread.

The list can't be a necessary part of the sentence.  This
means the sentence before the colon must be complete.

Incorrect:        
Items I need from the market are:  potatoes, eggs, and
bread.
(The first part of this sentence is not complete.  Imagine
the sentence,
Items I need from the market are.  This is not a complete
thought.)

* Between titles and subtitles

Colons separate the main title from a subtitle
Lord of the Rings:  The Two Towers

* Time

Colons separate the two parts of the time.
11:30, 12:15

* Salutations

Colons are used after formal salutations in letters or
e-mails.
Dear Jonathan Malone:

* Quotations

Colons are used to introduce a quotation.  This often
includes large
quotations separated from the rest of the text.
The United Nations made this statement regarding
poverty:

     ``quotation.....``
Learn English Pronunciation
 

Colons