Guy,
There are two main uses for the semicolon:
1) In place of a conjunction (like and or but):
"Time flies like an arrow; fruit flies like a banana."
2) As a separator in long or complex lists:
"We have offices in New York, New York; Dallas, Texas; Little Rock, Arkansas; and San Francisco, California."
There’s a third use, too, though nothing to do with grammar. In lists of e-mail addresses, it’s often used as a separator.
Altogether a very useful little puctuation mark.
- PW