It’s used instead of a conjunction to combine two sentences (
He did the crime; he must do the time). Don’t use a semicolon to connect a subordinate clause to a sentence ( Since he did the crime; he must do the time). In that case, use a comma.
In a sentence with a conjunction, don’t use a semicolon before the conjunction (
He did the crime; and he must do the time). Use either a comma or no punctuation before the conjunction. Or start a new sentence at the conjunction.
A semicolon also separates list elements with internal commas (We visited Tulsa, Oklahoma; Flint, Michigan; and Paducah, Kentucky).
Semicolons are often mistakenly used where a colon is correct, and vice versa. Make sure you’ve got the right one.