The semicolon is a much-misused, often maligned, but elegant and useful piece of punctuation. Neither a period nor a comma, the semicolon links in some instances and divides in others. There's no need to be afraid of it; there are only three main instances when you need to use a semicolon. Well, more like two and a half.
1. When you have two complete thoughts (independent clauses) that are closely enough related that they should be in the same sentence. How to tell if a semicolon is appropriate: You could put a period in and get two grammatically correct sentences. You can do this if you like, but if you want to indicate a closer relationship, put the two clauses in the same sentence with a semicolon between them. If you use a comma, you need to add a coordinating conjunction — and, but, or, etc. — or you'll end up with the dreaded comma splice.
For example:"I'm taking the bus to work this week; aliens ate my Buick."You could say "I'm taking the bus to work this week. Aliens ate my Buick." and each of those two sentences is grammatically correct. But the fact that you're riding the bus and giving the reason for that are connected, and best put in the same sentence. So you use a semicolon to do that.
2. Closely related to #1, when you have two independent clauses joined by a conjunctive adverb — however, nevertheless, moreover, and the like. Put a semicolon in before the adverb, and then use a comma after the adverb.For example:"Aliens ate my Buick; moreover, they ate every car on my street."
3. When you have a list of items that you would normally separate with commas and the list items contain their own commas. Use a semicolon to separate the items so the list is clear to your readers and not a train wreck of commas.For example:"Aliens ate my Buick, which was parked outside my house; the Nelsons' Ford, which was in their garage; two of the Millers' three Toyotas, the red one and the white one; and the Darby boy's toy tractor, which he had only just gotten from Santa."
If you read a lot of Austen or Dickens or other 19th-century writing, you may see semicolons used to separate coordinate clauses (usually containing commas) in long, complex sentences. It was the style at the time; and while we don't often encounter that usage today, having a preference for shorter, more succinct sentences, such a usage is not grammatically incorrect.
People run into problems with semicolons not using them where they need to (see above), and using them where a colon is needed instead: for the introduction of a list and after a salutation in correspondence.
Run from aliens, but embrace semicolons. They help make your sentences, and your meaning, clearer.