Admirals were high ranking officers and commanders of fleets in a naval force. The rank of Admiral is exclusive to the navy and is the equivalent of the army's General.
Typically, the main categories for Admiral were: Rear Admiral, Vice Admiral, and Admiral, in lowest to highest ranking respectively. The highest form of Admiral was also called a "Four Star Admiral" or a "Full Admiral".
Some consider the rank of Commodore (which only exists in few military groups) to be the most junior position in "Admiral", since it serves as the bridging rank between a navy Captain and an Admiral. In cases like these, the rank of Commodore is considered to be part of the "Admiral" category but is the lowest rank, being under the command of Rear Admiral.
In times of war, a Full Admiral could be promoted to the rank of Admiral of the Fleet (also known as: Fleet Admiral/Admiral of the Navy/Grand Admiral), which would then be considered the highest rank in an entire naval force.