A colon introduces an element or series of elements that illustrates or amplifies the information that preceded the colon. While a semicolon normally joins two independent clauses to signal a close connection between them, a colon does the job of directing you to the information following it.
The colon in this sentence signals that you are about to learn the names of the three types of muscles the sentence already mentioned. We might silently read the sentence this way.
Colons are often used with lists, as in the example above. They can also be used to signal further clarification.
One might silently read this as:
Colons can also introduce a quotation:
Colons Separating Independent Clauses
A colon can be used to separate two independent clauses when a) the second clause is directly related to the first clause (not just vaguely related) and b) when the emphasis is on the second clause. While you can also use a semicolon or a period between two independent-yet-related clauses, the colon is a little softer than the period, but a little harder than the semicolon.
In British English, the word following a colon is not capitalized unless it is a proper noun or an acronym. In American English, styles differ, but it is best to capitalize the first word after a colon if what follows forms two or more complete sentences.
Misuse of Colons
A colon should not separate a noun from its verb, a verb from its object or subject complement, a preposition from its object, or a subject from its predicate.
To illustrate, here is one of our sentences from above rewritten incorrectly.
Putting the colon here separates the verb are from its subject complements (cardiac, smooth, and skeletal).
The colon should be removed from this sentence because it separates the preposition to from its objects (Rome, Israel, and Egypt). To write this sentence correctly, the colon should be removed.