Forces & Newton's Laws

Mass and weight

Mass ($m$) is the amount of matter in an object, in kilograms (kg); it is the same everywhere.
Weight ($W$) is the gravitational force on an object, in newtons (N):

$$W = mg$$

where $g$ is the gravitational field strength ($\approx 9.8\ \text{N/kg}$ on Earth). Weight changes with location (less on the Moon); mass does not.