Semicolon ;

Basic Rule:  There must be an independent clause (complete sentence) on each side of a semicolon.

"It's supposed to rain tomorrow; we could use the rain."

"Call me tomorrow; we'll figure everything out then."

"This is Steve's book; I found it in my car."

Notice there are complete sentences on each side of the semicolons.  

If a period can't replace your semicolon, you didn't use it correctly.

Links

• Using Commas and Semicolons Correctly (quiz)

• How to use a Semicolon (theoatmeal.com)