The Colon (:)
It is used to indicate that what follows is additional information or clarification.
No space before it, and one space after it.
In British English, the word that follows does not usually start with a capital but American English normally does require a capital.
1) Separate the title and subtitle of a book or film eg: Star Wars: The Last Jedi
2) Citing Bible passages, do include uses spaces eg: Genesis 1:3-5.
3) Ratios 4:1 but write this out, four to one, when including information in formal writing
4) US English only - to separate hours from minutes when giving the time eg: 2:30, but British English would be 2.30