Back in the corner, next to the ever-disruptive Tommy, sits the semicolon. They are little bullies in their own special way, and our relationship with them is based more on fear than reality. Tommy uses physical threats; semicolons rely on ones more mental in nature—that if you use them 
incorrectly you’ll look grammatically foolish. One of the best mystery-removing descriptions of this offspring of the colon and comma states: The semicolon indicates a stronger pause than the comma but a weaker pause than the period. Of course that offers little help in determining when to use them.
Truth be told, our fear of semicolons, with their witchy hooked noses, is based mostly on a misunderstanding of their personalities. If they’re guilty of strong-arming anything, it’s forcing together two otherwise independent clauses (sentences) that could stand alone into a single structure without the need for a coordinating conjunction (and, but).
In a way, they’re space savers.
Take this example by writer Terry McMillan: “I didn’t call myself a poet; I told them I wrote poems.” The two clauses are fully capable of standing separately, but the semicolon links them for a smoother read. Whether you gasp or take a small quiet breath following the semicolon is probably a personal preference, but the pause is somewhere between that required by a full stop (period) and a comma.
But you can’t run around sprinkling semicolons all over your independent clauses. Semicolons should be confined to connecting independent clauses that are closely related to one another, i.e., deal with the same subject or topic, as in the MacMillan example above.
Like any rule of grammar, there is an exception: When the independent clauses are short, you can use a comma instead of a semicolon. Of course, “short” is not defined. “I woke up, I got out of bed.” Honestly, who would use that when “I woke up and got out of bed” or “I woke up and got up” works so much better. So consider the semicolon as another technique to add diversity to your writing.
Here’s another example, from Michael Strumpf and Auriel Douglas’ The Grammar Bible: “The chef at this restaurant, which I have never liked, makes terrible bouillabaisse; but we will eat wherever you like on your birthday.” In this sentence, the semicolon connects two related but more complicated independent clauses, each of which contains their own internal punctuation. In this case, the semicolon works very well, but you press the coordinating conjunction—but—back into service. Don’t be frightened. Mentally remove the but and read the sentence. Notice how it doesn’t sound quite right. That will tell you in more complicated sentences when a coordinating conjunction is needed.
And no, the dash IS NOT a substitute for a semicolon. Semicolons bring things together, dashes pry open a gap inside a sentence to insert an emphasized thought or topic, or to show there is an interruption. That’s not to say that you couldn’t take two sentences connected by a semicolon and convert them into something using a dash with a little rewriting. The Dick Tracy era has arrived; mobile phones have shrunk from a device you carry in a holster to something you wear on your wrist. Let’s rewrite it: Mobile phones have left the holster on our waists to become watch-sized devices we wear on our wrists—Dick Tracy style. Another alternative that further tightens the sentence and gets rid of the dash: Mobile phones have left the holster on our waists to become Dick Tracy devices on our wrists. When we used the reference to Dick Tracy as an interjected 
comparison, a sort of interrupter thought, we used a dash. We could probably use a comma, but note how the dash creates a sense of dramatic emphasis? Just don’t succumb to the tendency to over use it.
Sometimes the independent clauses that we want to join are more opposite than complementary. We then go back and rely on a conjunctive adverb to help us make our point. For example, Ann Taylor in “AIDS Can Happen Here” wrote: “I am faced with my imminent demise; therefore, life becomes a very precious thing.” The semicolon works perfectly because we are joining two independent and related clauses, but they work in contrast and thus require a conjunctive adverb that clarifies the distinction. The list of conjunctive adverbs include: also, besides, finally, however, indeed, instead, meanwhile, moreover, nevertheless, next, still, then, and thus.
Like so many other punctuation marks, the semicolon has been pressed into service elsewhere—to separate groups in a series that contain commas. This sentence, from Writers Inc (Write Source, 2001), is a great example, because it also shows an effective use of dashes: “Every Saturday night my brother gathers up his things—goggles, shower cap, and snorkle; bubble bath, soap, and shampoo; tapes, stereo, and rubber duck—and heads for the tub.”
So tame your fears and soon you’ll be using semicolons to add a little literary variety to your writing.
As for Tommy. Throw a couple of semicolons at him. If they don’t knock him down, they’ll probably confuse him. You don’t have to be afraid of him or semicolons anymore.
(Special Note: The above cartoon illustration is by Chris Krenzke, whose work has added a wonderful serendipity to the Write Choice collection of grammar manuals. Hey, wait a minute! Isn’t that Tommy?!)