Aside from certain conventional uses, a colon follows an independent clause to direct attention to a list, an appositive, or a quotation.
(list) You will need the following items for the trip: passport, change of clothes, money, and time.
(appositive) My dog is guilty of two of the deadly sins: gluttony and sloth.
(quotation ) Austen clearly ridicules John Dashwood: “He was not an ill-disposed young man, unless to be rather cold hearted, and rather selfish, is to be ill-disposed” (43).
Use a colon between independent clauses if the second summarizes or explains the first.
Faith is like love: it cannot be forced.
Conventionally, a colon follows the salutation in a formal letter, indicates hours and minutes, and separates the title and subtitle.
Dear Sir or Madam:
The New Pug: A Brief History of the Breed.