This is probably the most misused and misunderstood punctuation marks in the English language. What exactly is it, and what does it do?
First of all, it’s clearly a combination of a comma (,) and a colon (:).
A comma is used to separate elements of a sentence, such as items in a list, or clauses (a clause, generally, is a part of a sentence with its own sentence and verb). Most people use commas correctly without thinking about it, and the rules about them aren’t really strict anyway. Basically, you can use in a comma in a sentence where you would pause if you were speaking.
A colon is a little trickier and a little less common. A colon is used in the following ways: to indicate that what follows it explains, defines, describes, or lists elements of what precedes it. For example:
I have three sisters: Anne, Mary, and Sarah.
I saw his car: a dark green Ford Fiesta.
A colon is used in the following ways: to indicate that what follows it explains, defines or describes, explains or lists elements of what precedes it.
A basic way to think about it is that a colon usually introduces something.
What about a semicolon then? Its two most common uses are:
To separate two independent yet related clauses that aren’t joined by a contraction (e.g. and, but):
I like cats; she likes dogs.
To separate items in a list when some of those items contain commas within them:
Here are the options for dinner: steak, chips, and gravy; bacon, potatoes, and cabbage; and lamb, potatoes, and mixed vegetables.
As you can imagine, if one only used commas in the above list, it could be difficult to tell which food items went together. Actually, just look at that sentence for a second. Look at all that punctuation: a colon, semicolons, and commas (and perhaps even an Oxford semicolon)!
If all that’s just confused you more, then don’t worry: most people use colons and commas correctly instinctively.
And semicolons? As you can see, their usage is pretty specific. So much so, in fact, that you could probably blissfully live your whole life without ever needing to use one. In fact, I’d say don’t worry about them at all. Don’t force yourself to use them. Most of the time, people use them when they want to create a slightly longer pause in their reader’s mind than a comma would provide. Which isn’t really a bad way to use them, but isn’t strictly correct. And of course some people use them just to try to show off or to look clever: to show that they’re aware of more punctuation than full stops, commas and colons, even if they’ve no idea how to use it.
Probably the most common way you’ll use them is to separate items with commas in a list. And to be honest (*whisper*) you could probably get away without using them even then. There’s more than enough punctuation available to make your meaning clear without needing to use something like a semicolon.
Also, colon is another name for your large intestine, which is funny.