Colon:

A 

colon is used to indicate that what follows is a
demonstration
an example
or
A consequence of what is referred to before.

Example:
I know one thing: I am not going to Islamabad.

Enumerations:

To separate things which are to be mentioned one by one.
Example:
Please send photocopies of the following documents: your passport, your driving license and your birth certificate.

In scripts:

Colons are used for speech in scripts.

Examples:
Ali: how are You?
Ahsan: I am fine.
What about You?
Ali: I am fine too.
Ahsan: Good to learn.

Time:

To separate hours, minutes and seconds we use a colon.
Example:
The file was last modified at 12:35:10.

To separate chapters and verses:

A Colon is also used to separate verses and chapters in sacred books.
Example:
Sura 5:18.