The period is used at the end of a complete sentence, which can be a statement, command, or request. It is a visual marker to the reader that one complete thought has ended and that another may follow. In informal writing, periods can be used with single words.
Example:
Statement: Your bicycle is on my foot.
Request: Please move your bicycle off my foot.
Command: Get your bicycle off my foot.
Single words: Thanks. That feels much better.
Periods are also used in many abbreviations.
Example:
Saint: St.
Cash on delivery: C. O. D.
Avenue: Ave.
Organization: Y. U. O.
