Early in aviation history, pilots operated their aircraft by lying in the prone position directly on the wing and manipulating controls while exposed to the elements. But with the development of aviation, the way that pilots fly and operate an aircraft has grown more sophisticated while aircraft have advanced in both technology and design.
A cockpit is where a pilot and co-pilot will fly the aircraft by actively manipulating a control stick or yoke and a throttle. The cockpit is also where instrumentation about the aircraft's speed, heading and attitude are located. Small aircraft such as the Cessna 152 do not have a flight deck because the aircraft only have two seats and one deck.
Many large aircraft such as airliners have a flight deck. The flight deck is where the cockpit is located, but if the aircraft requires more than just a pilot and co-pilot to operate it, a flight or systems engineer might also be located there. The flight deck might also refer to the level on which the cockpit is located if the aircraft has two or more decks, such as the Boeing 747.
Aircraft Without a Cockpit
Although traditional aircraft will have at least a cockpit to operate and fly the aircraft, new technology has advanced aviation to the point where the cockpit can be located in another place than on the aircraft itself. The U.S. Air Force drone the Predator has no on-board cockpit. Instead, a pilot operates the aircraft via instruments from many miles away.
Other Aircraft Decks
In addition to the flight deck, and depending on the aircraft's function, an aircraft might have other decks that support different activities. In WWII, the B-17 Flying Fortress had a deck for navigation and radio operations that was separate from the flight deck. On a modern Navy P-3 Orion, this aircraft will have a deck dedicated to sonar in order to hunt for submarines.