When should a semicolon be used in a sentence

A semicolon is most commonly used to link (in a single sentence) two independent clauses that are closely related in thought. When a semicolon is used to join two or more ideas (parts) in a sentence, those ideas are then given equal position or rank.

Explanation:

Example: (1) Some people write with a word processor; others write with a pen or pencil. (2) I have a big test tomorrow; I can't go out tonight.