A semicolon marks a stronger break than a comma, but not as strong as a colon or period (full stop). Use a semicolon:
1) Between two related complete sentences or independent clauses that are not joined by a conjunction.
The Cubs needed a pitcher; they traded for Greg Maddux.
She had seen such weather conditions before; tornadoes were often the result.
2) In a series or clause that contains commas.
On 9/11, the US Capitol Police moved quickly to protect the congressional leaders: Tom Daschle, the Senate majority leader; Trent Lott, the Senate minority leader; Dennis Hastert, Speaker of the House; and Richard Gephardt, the House minority leader.