The colon has three main uses as a punctuation mark.
A colon is used to introduce a list.
Mary had four dogs: a labrador, a German shepherd, a boxer and an old mongrel called Jess.
There are four meals available to chose from at the event: pizza, curry, spaghetti bolognese and omelette.
A colon is not needed if the list is incorporated in the sentence.
The restaurant specialises in pizza, pasta and rice dishes.
A colon is used to separate two independent clauses when the second clause is an explanation or example of the first.
John could not go out on Friday night: he had no money.
At the start of the match there were three possible outcomes: we could win and be promoted or we could either lose or draw, in which case we would not.
A colon can be used to emphasise a word or phrase at the end of a sentence in a way which is more or less in line with the way the colon separates clauses in that the words after the colon provide an explanation of what has gone before.
Standing on the cliff edge there was only one thing that Butch and Sundance could do: jump.
As the aircraft was thrown about in the turbulence, there was only one place mum wanted to be: back on the ground.