Sir is an honorific address used as a courtesy title to address a man without using his given or family name in many English speaking cultures. It is often used in formal correspondence (Dear Sir, Right Reverend Sir).
The term is often reserved for use only towards one of superior rank or status, such as an educator or commanding officer, an elder (especially by a minor), or as a form of address from a merchant to a customer.
Equivalent terms of address are "ma'am" or "madam" in most cases, or in the case of a very young woman, girl, or unmarried woman who prefers to be addressed as such, "miss". The equivalent term for a knighted woman is Dame, or "Lady" for the wife of a knight.