The colon (:) is used less frequently than most other punctuation marks. It is used to introduce both lists and examples related to the preceding sentence. For example:
The following are important considerations when hillwalking: the weather forecast, the length of available daylight, and the steepness of the terrain.
The family had never been poorer: their cupboards were empty, their bank accounts were overdrawn, and they were facing eviction from their home.
A colon can be used to amplify what comes before it. For example:
The job was finished: after five long years of waiting, the house was ready.
It can be used to introduce a quotation. For example:
His reference email finished with my favourite bit: "He was worse than being a man short."