The semicolon is used in a series that has been introduced by a colon.
[There are three reasons he wants to go to Paris: he has never been there; his girlfriend is studying there; and his parents came from France.]
Otherwise, it is used in place of a period to join two sentences which are so close in meaning that a period would set them apart.
[My mother is a great hostess; people always want to be on her invitation listl.]