Mass vs Weight

• Mass measures of how much stuff an object has; it is an intrinsic property that doesn't depend on where the object is. Mass is measured in kilograms (kg).
  
  Weight

  $W$

  Units:Newtons (N)
• Weight, on the other hand, is the force of gravity on the object; it only makes sense when there is something (like the Earth) creating a gravitational field. Weight is measured in Newtons (N).

On Earth, those two values are proportional by the equation $$W=mg$$ so it's common to use the term interchangeably. Saying "I weigh 100kg" might be technically incorrect (because weight is in Newtons), everyone knows what it means. * The same is not true when you leave the surface of the Earth, however. On the moon, your weight would be about one-sixth its value on Earth, because the moon has a smaller gravitational pull ($g=1.6\mathrm{N}/\mathrm{kg}$) than Earth does, but your mass would stay the same. And in interstellar space, a pebble and an asteroid would both have zero weight, and yet the asteroid clearly have a larger mass (i.e. more stuff) in it.

Footnote

Weight and mass are so interchangeable that people often convert kilograms directly into pounds when talking to Americans, even though the pound is actually a measure of force, not mass. (You might be familiar with the unit "pounds per square inch" or PSI, where it's more obvious that we're talking about force.)

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).