They're used interchangeably in everyday speech and I lose marks for it in science.
Accepted answer
**Mass** is how much matter an object contains. Measured in **kilograms**, with a balance. It's a property of the object and **doesn't change** with location.
**Weight** is the *force* gravity exerts on that mass. Measured in **newtons**, with a newton-meter (spring scale). It **does** change with location, because it depends on the gravitational field strength:
**W = m × g**
On Earth `g ≈ 9.8 N/kg`, so a 50 kg student weighs about 490 N. On the Moon `g ≈ 1.6 N/kg`, so the same student — still 50 kg of matter — weighs about 80 N.
That's the standard exam question: going to the Moon changes your weight, not your mass. An astronaut in orbit is often called "weightless," but their mass is unchanged; they're in continuous free fall.