Mass is a measure of how much stuff makes up that object; it is measured in kilograms (kg). Weight is the force of gravity on an object; it is measured in Newtons (N).
On earth, those two values are so closely linked by the equation \(W=mg\) that we use the two terms interchangably, so saying "I weigh 100kg!", while technically incorrect, is clear to the listener.
If we were to go to the moon, on the other hand, our weight would decrease to one-sixth of its value on Earth, because the moon has a smaller gravitational pull than Earth does. Our mass, however, would stay the same, even if we were in deep space with no planets or stars around, and no sources of gravity.
Note that the American pound is actually a unit of force, not mass, so that it is correct to say "I weigh 220lbs". While most people will give the conversion factor of 1kg=2.2lbs, this in fact only works on Earth, because the two units measure entirely different things. The 220lb person would weigh 37 pounds on the moon, but still be 100kg. This slug has a mass of a few millislugs...
The American unit of mass, which is infrequently used, is called the slug, which is equal to 14.6kg. One slug weighs about 32lbs on Earth, so if you give your weight in slugs the number will sound a lot lower (albeit slimier).
