The colon (":") is a punctuation mark, simply consisting of two equally sized dots centered on the same vertical (up/down) line.

Punctuation

Use in prose 
A colon is a more significant pause than a semicolon. It is usually used to contrast two parts of a sentence:
It's official: McClaren makes the worst start by an England manager.
When the door was forced,  a scene of chaos was revealed: chairs overturned, drawers pulled out and emptied, broken crockery on the floor...
If you must  go, take the following: climbing rope, ice axe, compass, a large-scale map, emergency water and food, and good boots.
Man proposes: God disposes.
The Lord seeth not as man seeth: for man looketh on the outward appearance, but the Lord looketh on the heart.

Use in other kinds of text 
 Introduction of a definition, such as:
A: the first letter in the Latin alphabet
Hypernym of a word: a word having a wider meaning than the given one; e.g. vehicle is a hypernym of car

 Separation of the chapter and the verse number(s) indication in many references to religious scriptures, and also epic poems; it was also used for chapter numbers in roman numerals, as in:
John 3:14–16 (or John iii:14–16) (cf. chapters and verses of the Bible)
The Qur'an, Sura 5:18

 Separation when reporting time of day hour/minute/second (cf. ISO 8601), such as:
The concert finished at 23:45.
This file was last modified today at 11:15:05.
 Separation of a title and the corresponding subtitle, as in:
Star Wars Episode IV: A New Hope
 Separation of clauses in a periodic sentence
 Colons can also be used to start a list, such as, "He provided all of the ingredients: sugar, flour, eggs, and butter."

History 
The colon's first appearance in English text is marked by the Shorter Oxford English Dictionary as 1589.

Diacritical usage 

A special double-triangle colon symbol is used in IPA to indicate that the preceding sound is long. Its form is that of two triangles, each a bit larger than a point of a standard colon, pointing toward each other. It is available in Unicode as modifier letter triangular colon, Unicode U+02D0 (). A regular colon is often used as a fallback when this character is not available, or in the practical orthography of some languages (particularly in Mexico), which have a phonemic long/short distinction in vowels.

Mathematics 
The colon is also used in mathematics, cartography, model building and other fields to denote a ratio or a scale, as in 3:1 (pronounced "three to one"). A betting odd of the form  corresponds to the probability .  Unicode provides a distinct ratio character, Unicode U+2236 () for mathematical usage.

In many non-English-speaking countries, the colon is used as a division sign: "a divided by b" is written as a : b.

The combination with an equal sign, , is used for definitions.

Computing 
In computing, the colon character is represented by ASCII code 58, and is located at Unicode code-point U+003A. The full-width (double-byte) equivalent, ：, is located at Unicode code point U+FF1A.

The colon is quite often used as a special control character in many operating systems commands, URLs, computer programming languages, and in the path representation of several file systems. It is often used as a single post-fix delimiter, signifying the immediate precedence of a token keyword or the transition from one mode of character string interpretation to another related mode. Some applications, such as the widely used MediaWiki, use the colon as both a pre-fix and post-fix delimiter.

For a double-colon, "::" the meaning has included the use of ellipsis, as spanning over omitted text; however, there have been other meanings as well.

Internet usage 
On the Internet (online chats, email, message boards, etc.), a colon or multiple colons is sometimes used to denote an action or emote. In this use, it has the inverse function of quotation marks—denoting actions where unmarked text is assumed to be dialog. For example:

Kim: Pluto is so small, it should not be considered a planet. It is tiny!
Mel: Oh really? ::Drops Pluto on Kim’s head:: Still think it's small now?

Colons may also be used for sounds (as with ":Click:"). One can contrast this use with the use of outer asterisks (for example, *cough* would denote that the speaker is coughing, as opposed to saying the word 'cough').

It also has the widespread usage of representing two vertically aligned eyes in a emoticon, such as :-), :( :P, :D, :3, etc

Related pages
Semicolon, the ";" punctuation mark

References 

Punctuation
Typography