Colons formally introduce a list or series, a question, or an amplification.

We test three types of collectors: flat plates, evacuated tubes, and parabolic troughs.

When the colon is followed by a complete sentence, capitalize the first letter of the first word after the colon.

The relationship between the two is inverse: When volume increases, pressure decreases.

Colons also often separate the parts of a ratio.

We added enough water to obtain a 3:1 dilution.

However, commas, not colons, usually follow words such as "that is," "namely," and "for example." You don't need a colon after a verb or preposition that precedes or introduces a list ("includes," "to," "with," "between," etc.). Use a colon when a noun (such as "the following") introduces a list in the text.