I do a Grammar Fridays on my blog, and this just happens to be the topic I covered last week.
A semicolon is used to join to independent
clauses which are very similar or very different (to show contrast.) It is used instead of a period and shows a tight relation between the two sentences.
Example: He didn't eat a slice of bread; he ate the entire loaf.
A semicolon is NEVER to be used before a conjuction (but, and....). A semicolon can also be used as a 'super' comma in lists:
I visited Paris, France; Rome, Italy; Berlin, Germany; and Amsterdam, Netherlands. (This is a really simple example, but should give you the idea)
Hope that helps!