Semicolon

1. Separates the clauses of a compound sentence having no coordinating conjunction: Do not let us speak of darker days; let us rather speak of sterner days.—Winston Churchill
2. Separates the clauses of a compound sentence in which the clauses contain internal punctuation, even when the clauses are joined by conjunctions: Skis in hand, we trudged to the lodge, stowed our lunches, and donned our boots; and the rest of our party waited for us at the lifts.
3. Separates elements of a series in which items already contain commas: Among those at the diplomatic reception were the Secretary of State; the daughter of the Ambassador to the Court of St. James's, formerly of London; and two United Nations delegates.
4. Separates clauses of a compound sentence joined by a conjunctive adverb, such as however, nonetheless, or hence: We insisted upon a hearing; however, the Grievance Committee refused.
5. May be used instead of a comma to signal longer pauses for dramatic effect: But I want you to know that when I cross the river my last conscious thought will be of the Corps; and the Corps; and the Corps.—General Douglas MacArthur

See also: The Semicolon.