Answer:
The force that gravity exerts on a given mass.
Gravity = 9.8 m/s^2
F = M*A
Weight = Mass * Gravity
That is why your WEIGHT is different on the moon, but your mass will always be the same.
Mass is a scalar, hence only has a magnitude (kilograms, kg). Weight is a vector (its a force - measured in Newtons, N) and so also has a direction. If your answering a weight question give a direction. Usually 'down' will suffice.