In the
Navy of most countries, a Rear Admiral is a rank just below
Vice Admiral, two levels below plain
Admiral. The origin of the term is that the Vice Admiral's assistant in the
British navy was (at the time) the least important of the
flag officers and commanded the
reserves and the rear ships of the
fleet instead of being up front or in the middle of the fleet with the higher admirals. Hence, the name "Rear Admiral." This term was in use by the seventeenth century.
In the U.S. Navy, Rear Admiral was the first Admiral rank created, in 1862 during the American Civil War; no Admiral rank had been used before because the term was felt to be too reminiscent of royalty. It was two years later before any higher rank was created in the U.S. Navy.
In the U.S. system of ranks, the rank of Navy or Coast Guard Rear Admiral is divided into "upper half" and "lower half" which are the equivalent of an Army, Air Force, or Marines Major General or Brigadier General.
Sources:
http://www.history.navy.mil/trivia/triv4-5l.htm
http://www.tesarta.com/www/resources/library/militaryrank.html