The Essentials of Writing

The Hamilton College Style Sheet

Essentials of English Usage

Punctuation ("p")

Every sentence must end with the correct terminal punctuation -- a period, a question mark, or an exclamation mark.

For thorough discussion of punctuation within sentences, see the following:
Comma Splice ("cs"), Fragment ("f"), Run-on ("r"), Semicolon ("semi"), and Subordinate Elements, Punctuation of ("ps").  For more information on punctuation, refer to Punctuation Patterns and The Second Deadly Sin: Incorrect Punctuation of Two Independent Clauses.