1. Semicolons are used to separate items in a list when the items already contain commas.
Normal List of items:
Inside the basket of fruit there was an apple, an orange, a banana, and a bundle of grapes.
List requiring semicolons:
Inside the basket of fruit there was a juicy, red apple; a shrunken, unripe orange; a half-eaten banana; and a bundle of overripe grapes.
Note: The last item has no commas, but a semicolon was used anyway. In mixed cases, I think, but have been unable to confirm, that you should use a semicolon if it makes the list easier to read. Just make sure you don't mix commas and semicolons within the same list. Be consistent.
2. Join independent clauses with a semicolon when one or both clauses contain a comma (Rozakis 156).
For example:
Wickham, a skinny kid with a perpetually hungry look, left his food, even though he hadn’t had enough; but George, who was full, couldn’t be blasted free from his plate—some people simply have stronger stomachs than others.
Sources
Rozakis, Laurie. English Grammar for the Utterly Confused. McGraw-Hill Companies, Inc., 2003.
