Weight or Mass?
Q: Aren't "weight" and "mass" the same?
A: Not really.

An object has mass (say 100 kg).
This makes it heavy enough to weigh 100 kg 
Gravity causes Weight

An objects weight is how hard gravity is pulling on it.
We think the weight is the same everywhere ... because we all live on the surface of the planet Earth! 
But if the object were far out in space it would just float around, exerting no force on the scales.
The scales would show 0 kg ... but the mass is still 100 kg !


An object's mass doesn't change (unless you remove some!), but its weight can change.
So Why Do People Say Weight instead of Mass?
People often use "weight" to mean "mass", and vice versa.
Because gravity is pretty much the same everywhere we go we don't notice a difference.
But remember .. they do not mean the same thing,
and they can have different measurements.
Here are some conditions where the Weight might change:
 in space (can be weightless!)
 on the moon (a 100 kg mass would weigh 16.6 kg)
 you can even get very slight differences in weight in different locations on earth!
Weight is a Force
So ... if weight and mass are different, why are they both in kilograms?
Well, weight should not really be in kilograms!
I have used "kilogram" so far because that is what you would see on a pair of scales, but it is technically wrong to talk about weight in kilograms ...
... so sometimes people say "kilogram force" (kgf) or "pound force" (lbf) to show that they are talking about the force that the mass exerts because gravity is pulling down on it.
But there is a better measurement ... Newtons
Newtons
The correct unit for force is the Newton (1 kg·m/s^{2}) which is abbreviated N.

Gravity makes a 1 kilogram mass exert about 9.8 Newtons of force 
So a 100kg mass really weighs about 980 Newtons.

Why Scales Show Kilograms or Pounds
But scales show Kilograms or Pounds because that is what people understand best ...
... but it is really just an estimate of the mass above them.
Scales should really show Newtons, but that might confuse people!
Question: how many Newtons should the scales show when you stand on them (hint: multiply kg by 9.8)? 
 So the scales show an estimate of your mass based on the force your body exerts on it.
 And to find out how much force your body is exerting on the scales, multiply by 9.8 (to convert kg into Newtons).
Apparent Weight
But Scales can be fooled ... because they measure a "downwards force" and don't know if it is gravity or some other force!
Just jump up and down (gently!) on your scales at home to see your apparent weight change, while your mass stays the same.
So your mass is the same, your weight is the same (because the force of gravity hasn't changed), but your "apparent" weight changes. Read more at Apparent Weight
Conclusion
 Mass is a measure of how much matter something contains
 Weight is a measure of how strongly gravity pulls downwards
 Apparent Weight is a measure of downwards force
 Force is measured in Newtons, not kilograms or pounds
 When scales show "kg" or "lb" it is just an estimate of the mass above them
