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Mass is the quantity of matter which a body contains. This is the same wherever the body is. It is measured in kg.

There are three states of matter: solid, liquid and gas - all of these contain atoms and/or molecules that make up the mass of the object.

If my mass was 65 kg. This would be true wherever I went in the universe. Just because I travelled to the moon wouldn't make the number of atoms and molecules in my body change... it would be the same... just as my dress-size would! My mass would remain at 65 kg.
Mass AlwaysStays theSame!!

The force that acts on the mass of an object because of gravity is called its weight. Therefore weight is a force and is measured in newtons.

Weight is the force of gravity acting on a mass that is positioned in a gravitational field. This changes with the strength of the gravitational field.

There is an equation that links mass to weight in a gravitational field:

 
w = mg
w
 = the weight (in newton)
m
 = the mass of the object (in kilogram)
g
 = the gravitational field strength at the position where the mass is placed (N/kg)

 

On Earth g = 10N/kg. Therefore my weight is 650 Newtons.

This is only true whilst I stay on Earth. On the moon, I would be very light and jumping would be very easy. This is because g (Moon) is only 1.67 N/kg. I would weigh a sixth of what I did on Earth, but it wouldn't mean I was any thinner.. I'd still have all those atoms and molecules that make up my mass!