Colon:

It is a punctuation mark(:) used to:

to introduce a list of items:

These are our options: we go by train and leave before the end of the show, or we take the car and see it all.

in formal writing, before a clause or phrase that gives more information about the main clause:

You can use a semicolon or a full stop, but not a comma, instead of a colon here()

The garden had been neglected for a long time: it was overgrown and full of weeds.

To introduce a quotation, this may be indented:

As Kenneth Morgan writes

The truth was, perhaps, that Britain in the years from 1914 to 1983 had not changed all that fundamentally.

Others, however, have challenged this view