The semi colon ( ; ) is a punctuation mark. It is different from the colon, which has two dots, by a comma instead of a dot at the bottom. Before we analyze when exactly to use semi colon let's first look at its general usage:

• Semi colon is used to separate words of opposed meaning
• Semi colon is used to indicate interdependent statements.

Semi colons are usually followed by a lower case letter. They are not followed by a lower case letter if that letter is the first letter of a proper noun (e.g., USA). Semi colons have no spaces before them, but one space after (can be two in a monospace type). There are different applications of the semi colon.

English language applications

Between closely related independent clauses not conjoined with a coordinating conjunction

"I went to the tennis court; I was told it was closed for cleaning."

"I told Jim he's running for the hills; I wonder if he knew I was joking."

"Nothing is true; everything is permitted."

"A man chooses; a slave obeys."

Between independent clauses linked with a transitional phrase or a conjunctive adverb

"I like being odd; yet, I hate being different."

"I like to eat fish; however, I don't like to be eaten by them."

Between items in a series or listing containing internal punctuation, especially parenthetic commas, where the semi colons function as serial commas:

"Examples of familiar sequences are: one, two, and three; a, b, and c; and first, second, and third."

"She saw three men: Jamie, who came from New Zealand; John, the milkman's son; and George, a gaunt kind of man."

"Several fast food restaurants can be found in each of London, England; Paris, France; Dublin, Ireland; and Madrid, Spain."

Other applications

In computer programming, the semi colon is often used to separate multiple statements.

In mathematics, it is used to separate variables and parameters.

Sources:

History of the semi colon

New York Times: Celebrating the Semi colon in a Most Unlikely Location

Wikipedia: Semicolon

The Guardian: The end of the line?