The colon ( : ) is a punctuation mark consisting of two equally sized dots centered on the same vertical line. A colon precedes an explanation or an enumeration, or list. A colon is also used with ratios, titles and subtitles of books, city and publisher in bibliographies, biblical citations between chapter and verse, and for salutations in business letters and other formal letter writing, and often to separate hours and minutes.
This is a companion discussion topic for the original entry at http://grammarerror.com/tags/colon