View source for Tanker (Air Refueling)

A tanker is an aircraft used for aerial refueling.

Tankers are considered "force multipliers," because they convey considerable tactical advantages. Primarily, aerial refueling adds to the combat radius of attack, fighter and bombers aircraft, and allows patrol aircraft to remain airborne longer, thereby reducing the numbers of aircraft necessary to accomplish a given mission. Aerial refueling can also mitigate basing issues which might otherwise place limitations on combat payload. Combat aircraft operating from airfields with shorter runways must limit their takeoff weight, which could mean a choice between range (fuel) and combat payload (munitions). Aerial refueling, however, eliminates many of these basing difficulties because a combat aircraft can take off with a full combat payload and refuel immediately.

Aside from these issues, the psychological advantage of full fuel tanks – and a tanker likely available nearby – gives a pilot a distinct edge in combat. In most combat situations, speed is a necessity for optimal completion of the mission at hand. As high speeds require fuel, pilots must always balance fuel and speed requirements. Pilots operating aircraft with aerial refueling capability eliminate low-fuel concerns.

http://en.wikipedia.org/wiki/Aerial_refueling