A semicolon is a punctuation mark that shows you there's a pause in a sentence; semicolons separate two complete clauses.
Unlike a comma, which also indicates a pause in a sentence, a semicolon is used between related independent clauses. This means wherever there's a semicolon, it can be replaced with a period; however, this results in more abrupt, shorter sentences. You can also usually replace a semicolon with a comma followed by a conjunction like and, for, or but. The first printed semicolon appeared in 1494.