Use a colon (:) to give more information about something mentioned previously in the sentence. For example:
- He blamed his divorce on one thing: beer.
Use a semicolon (;) when a slight break is preferable to starting a new sentence. (A semicolon gives a smoother transition than a full stop/period.) For example:
- The pilot was worried; the elevators were packed with ice.
Use three dots (…) as a pause for effect. For example:
- Crime does not pay … as well as politics.
If you can't remember any of the rules above, use a dash (—). It covers all three.
Using Colons 
A sentence can be extended with a colon when the writer wishes to expand on something already mentioned in the sentence. (A colon is used to introduce some more information about something previously mentioned in the sentence.) For example:
- There were two pets in the house: a budgie and a cat.
(two pets: a budgie and a cat)
Using Semicolons
A sentence can be extended with a semicolon when a slight break is preferable to a new sentence.For example:
- No one was hurt in the accident; the only real injury was a broken finger.
- She did not approve of my design at all; she found it crass.
Quite often, there is a short phrase immediately after the semicolon that acts like a bridge between both halves of the sentence. It is called a transitional phrase. For example (with the transitional phrases shaded):
- Everybody knows he is guilty; however, it will never be proven.
- Her own guest was declined; as a result, she left.
- This business will collapse if you do not invest in the staff's well-being; of course, that is just my opinion.
Using Three Dots
If you want to create pause for effect, use three dots. For example:
- I don't want to achieve immortality through my work…I want to achieve it through not dying. (Woody Allen)
- She had a bath once a year…whether she needed it or not. (Mark Twain)
- As I moved the bushes, I was not confronted by the deer I had been tracking for two days but…a Bengal tiger.
Using Dashes
Confused about colons, semicolons, and three dots? Use a dash. The dash performs all the functions of the colon, the semicolon, and three dots mentioned above.
Examples:
- He blamed his divorce on one thing – beer.
(replaces a colon)
- No one was hurt – the only injury was a broken finger.
(replaces a semicolon)
- As I moved the bushes, I was not confronted by the deer I had been tracking for two days, but – a Bengal tiger.
(replaces three dots)
