What is the difference between weight and mass?

Mass measures the quantity of matter in an object and is an intrinsic property, identical everywhere in the universe, measured in kilograms or grams. Weight is a force, specifically the pull of gravity on that mass, measured properly in newtons, and it varies with the strength of the gravitational field you happen to be standing in. On the Moon, where surface gravity is about one sixth of Earth's, your weight drops to a sixth while your mass is unchanged, which is why astronauts bounce without having become smaller.

Everyday language ignores this entirely, and mostly gets away with it. Bathroom scales report kilograms or pounds, which conflates the two, and that works fine because everyone using them stands in essentially the same gravitational field. The distinction becomes real in physics, engineering, and space travel, and it surfaces in a subtle practical way: a balance scale comparing an object against known masses measures true mass and gives the same answer anywhere, while a spring scale measures force and would read differently on the Moon. The pound is also ambiguous in ordinary use, since pound-mass and pound-force are distinct units that happen to coincide numerically at Earth's surface, which is exactly the kind of unit confusion that has caused real engineering failures, including the loss of a Mars orbiter to a metric-imperial mismatch.