Definition of Subject Headings

Subject headings are a type of controlled vocabulary that  is used to take the guesswork out of searching by using a single term to describe a subject.

For example, the American Revolutionary War can be described using the following terms:
American Revolution
American Revolutionary War
Revolution, American
Revolutionary War, American
War of the American Revolution
American War of Independence
War of American Independence
War of Independence

By using a controlled vocabulary, all materials about the American Revolutionary War are listed under one heading:
United States -- History -- Revolution, 1775-1783

Having a controlled vocabulary helps create "see also" references :
American Revolution -- See -- United States -- History -- Revolution, 1775-1783

It also creates consistent subdivisions:
United States -- History -- Revolution, 1775-1783 -- Battlefields
United States -- History -- Civil War, 1861-1865 -- Battlefields
World War, 1914-1918 -- Battlefields

Using a standardized controlled vocabulary allows searches to be done quickly and with more accuracy. Also, since most library's use the same controlled vocabulary in their catalogs, only one vocabulary style needs to be learned.

Controlled vocabularies are are used outside of the library, with a common example being the  yellow pages in a phone book. For example:

See also reference:
Lawyers -- see Attorneys

Subject heading:

Attorneys -- Personal Injury  & Wrong Death

Definition of Subject Headings
Library of Congress Subject Headings
Types of Subject Headings
Types of Subdivisions
Example LCSH Entry