What is empty weight?
Total weight of an aircraft, excluding crew, payload, useable fuel and drainable oil.
What is gross weight?
Total aircraft weight at any moment during operation. Will decrease during flight due to fuel consumption.
Fuel weight?
Weight of the fuel in the aircraft at any given time.
Maximum weight?
Weight limits that cannot be exceeded during specific configurations or operations.
Useful load?
The empty weight minus maximum takeoff weight. This number is the max amount of weight you can add to the aircraft