Mary Norris writes about the subtleties of the semicolon for The New Yorker:
I think the semicolon is more easily understood if it is defined in relation to the colon rather than to the comma. Under “Semicolon,” the book says, “Its main role is to indicate a separation between two parts of a sentence that is stronger than a comma but less strong than dividing the sentence in two with a full stop…. She looked at me; I was lost for words.”
So the semicolon is exactly what it looks like: a subtle hybrid of colon and comma. Actually, in ancient Greek, the same symbol was used to indicate a question.
As a writer and English nerd, I love articles like this.