The semicolon seems to be a feared bit of punctuation. Most don't know how to use it, and if they do know how to use it, they're afraid of it. It's not a colon. A colon is a big deal. It adds emphasis, and it announces lists. The colon can make the leap from sentence to mathematics, where it announces a ratio.
But how do we use it in language? How do we use it in writing?
Don't feel bad if you don't know how to use a semicolon. And no, grammarian wannabe, I don't mean badly, I mean bad. As in "bad person." You're not a bad person if you don't know how to use a semicolon.
Think of the semicolon this way--the way that finally worked for me: it's a hinge. A semicolon is like a hinge in that it can join two sentences very closely together, as a hinge on a door joins the door to the frame.
A semicolon is a marriage between the period and the comma. It's a partial stop. The acid test for the semicolon is whether or not the two parts of the joined idea can stand on their own. Take a look at this example:
1. I like semicolons. They're cool.
Two separate ideas. The way the example is written, we read two things. Two separate things, joined only by proximity.
But...if we want to speed it up and express the point in a subtle way, example two comes in:
2. I like semicolons; they're cool.
If I wanted to make a bigger deal of it, I would whip out the colon:
3. I like semicolons: they're cool.
There is a certain dubiousness to the semcolon, cast upon it by English teachers--the dusty ones who don't want to argue punctuation as style. Don't get mad at them for it. They're mostly human and have limited time to spend on the more subtle pieces of punctuation because they're too busy fighting the run-on sentence to give students the out by teaching that a semicolon exists, yes, exists, to make the run-on sentence a piece of good writing.
*I salute E.B White who, in addition to his famous book Charlotte's Web, wrote Elements of Style, a little grammar book that explains things like how to use a semicolon. Some people harried good old E.B for writing the book. Those people were pretty successful in burying the little book. Those people were grammarians who want to apply rules to communication by regulating punctuation and other rule-ish stuff like that. Those people, in their anal retentiveness, obviously couldn't read the title and didn't know the difference between grammar and style.