Colons have a certain amount of uses, but what they mostly have in common that the colon is used to connect what precedes it with what follows. The two dots of a colon as if they were stretched out to form an equal sign will help you so that you get cases like this: "he provided all the ingredients: brown sugar, milk, butter, and eggs." A colon separates the city from the publisher. For example, "New York: New Directions, 1903." Colons help separate chapters from verses too. For example, John 14:27.  It also separates minutes from hours in times of day when given in figures: "10:00."

It is incorrect to substitute a semicolon in any of these cases. The semicolon will be erecting a little barrier with that dug-in comma under the dot. Semicolons always imply separation rather than connection. A sentence made up of two distinct parts whose separation needs to be emphasized may do so with a semicolon. For instance, "Cynthia moved to Washington; she was sick of getting picked on in Los Angeles." When a compound sentence contains commas within one or more of its clauses, you have to escalate to a semicolon to separate the clauses themselves. For example, "It was a sunny, deliciously warm spring day; Mary decided to walk to the fair." The other main use of semicolons is to separate one series of items from another; a series within a series, if you will. "The issues discussed by the board of directors were many: the loud, acrimonious complaints of the stockholders; the abrupt; devastating departure of the director; and the startling. Humiliating discovery that he had absconded with half the company's assets. Any time the phrases which make up a series contain commas, for whatever reason, they need to be separated by semicolons.

Many people are so terrified of making the wrong choice that they try to avoid colons and semicolons altogether, but I'm afraid this just can't be done. Formal writing requires their use, and it's necessary to learn the correct patterns.

                The Uses of Colons and Semicolons