What is the difference between Airbus 300 Airbus 340 and Airbus 380?

Size and Number of Engines

The A300 is a 2-engine passenger/cargo jet. The A300B2 is short-ranged; the A300B4 is short- to medium-ranged; the A300-600 and -600R are medium-ranged. The A310 is a much shorter variant that has a longer range than the A300, and like the -600 has a newer electronic cockpit.

The A340 is a much longer, 4-engine aircraft derived from the A300. It was designed to be immune to FAA regulations on two-engine planes. It failed to meet Airbus' expectations because it was undermined by rapidly-improving twinjets such as the B767, B777, and A330, after the FAA started allowing two-engine aircraft to fly longer routes.

The A380 is the new super-jumbo jet that has 2 cabins (or 2 floors). It is Huge with a gross weight that tops 1,000,000 lbs. Its high capacity, long range, and higher fuel efficiency per seat than (every single iteration of) the Boeing 747 is intended to break Boeing's monopoly on the high-capacity-airliner market. It's so big, though, that it needs a longer takeoff/landing run and terminals also need to be modified to accommodate its larger wingspan, as well as adding that extra upper-deck jetway that wasn't really needed for the smaller B747. Because of this, many airlines still prefer the 747. The A380F was a planned cargo version of the A380, but it was postponed until production of the A380 stabilized.