The colon (:) seems to bewilder many people, though it's really rather easy to use correctly, since it has only one major use. But first please note the following: the colon is never preceded by a white space; it is always followed by a single white space in normal use, and it is never, never, never followed by a hyphen or a dash — in spite of what you might have been taught in school. One of the commonest of all punctuation mistakes is following a colon with a completely pointless hyphen.
The colon is used to indicate that what follows it is an explanation or elaboration of what precedes it. That is, having introduced some topic in more general terms, you can use a colon and go on to explain that same topic in more specific terms. Schematically:
More general: more specific
A colon is nearly always preceded by a complete sentence; what follows the colon may or may not be a complete sentence, and it may be a mere list or even a single word. A colon is not normally followed by a capital letter in British usage, though American usage often prefers to use a capital. Here are some examples:
Very occasionally, the colon construction is turned round, with the specifics coming first and the general summary afterward:
While you're studying these examples, notice again that the colon is never preceded by a white space and never followed by anything except a single white space.
You should not use a colon, or any other mark, at the end of a heading which introduces a new section of a document: look at the paragraphs of this section.
The colon has a few minor uses. First, when you cite the name of a book which has both a title and a subtitle, you should separate the two with a colon:
Second, the colon is used in citing passages from the Bible:
Third, the colon may be used in writing ratios:
Finally, the colon is used in formal letters and in citing references to published work.