I was flicking through my physics textbook (as you do when you need to revise for a test that is going to decide your grade of the whole year), when a certain question caught my eye it read: Explain the difference between a mass of 100g and a weight of 1newton.
The mass is an intrinsic property of an object, weight is the force with which that object pushes the ground (depends not only on the object but on the gravitational force in it's vicinity as well).
On the moon the mass of that object will stay the same, but it's new weight will be 1/6 of it's weight on earth because moon's gravity is much weaker.
Mass is a scalar. Weight is a vector.
100 grams indicates the amount of matter in an object, and is a measure of the object's inertia, its resistance to acceleration. At relativistic speeds, or when dealing with sub-atomic particles, mass also is a measure of the amount of energy in an object.
1 Newton is a measure of the force exerted on an object by a gravitational field, or equivalently by acceleration. The weight vector of an object on Earth points toward the center of the Earth. If weight is caused by acceleration, the weight vector points opposite from the direction of acceleration.
A mass of 100 grams has no directional component, whereas a weight of 1 Newton has both directional and mass components. It is the directional force necessary to accelerate 1 kilogram of mass at the rate of 1 meter per second, per second (1Kg * 1m/sec^2).
These differences are important to avoid confusion in dimensional analysis. If you say that 100 grams is one tenth of 1 Newton, that would be comparing apples to oranges.