Colon vs Semicolon

Someone was asking about this earlier, and it just so happens that I have recently brushed up on my colon/semicolon usage.  So here's my answer:

It hinges on the relationship between the two clauses.

A colon is used when the text that follows is "further to" the current text.  It can be used to off-set both dependent and independent clauses, but the information has to follow on from the clause that was before it.

It's used when you introduce information:

There is something you must know: John is an inveterate liar.

Or when you give a list (after implying you are about to give a list):

There are three things you should know about John: he's ugly, he smells bad and his mother dresses him funny.

Or when there is a clear causal relationship (or you want to suggest there is):

John's lying ways would eventually catch up with him: he died alone in an abandoned mining town.*

In common usage, the colon is also used to introduce an example or a chunk of text (like I have done above).  Stylistically, though, you shouldn't use it for headings in documents.

A semi-colon is used to join two independent clauses that could easily be separate sentences, but are linked to each other on some level.

For example, if I wrote:

Your father is agitated today.  I saw John at the shops.

Those two sentences could be connected, or they could just be separate pieces of information that just happen to be next to each other.

But, if I wrote:

Your father is agitated today; I saw John at the shops.

Then the semicolon indicates that the two clauses are connected to each other, and one should be read in the light of the other.

Now, I could have written one of the sentences from my colon examples with a semicolon:

John's lying ways would eventually catch up with him; he died alone in an abandoned mining town.

And doing so would actually change the relationship between the clauses.  When I used a colon, I was indicating that one thing definitely lead to the other.  By using the semicolon, though, I'm hedging.  The death is connected to the lying ways, but not necessarily the result of them.

That's the usage side of things.  The style side of things (in terms of how many spaces to leave after the punctuation mark and whether the next word starts with upper or lower case) is a bit messier.  I have seen some style guides recommend that you treat a semicolon like a comma and a colon like a full stop, but other guides have suggested treating them both like a comma.

The answer is to hide behind the cloak of invisibility that is "consistency".  Pick a pattern and stick with it, until told otherwise.