What is the difference between mass and weight?

Mass vs Weight — commonly confused:

Mass: amount of matter in an object; measured in kilograms (kg) or grams (g); constant — same on Earth, Moon, or in space; measured using a balance (compares to known masses).

Weight: gravitational force on an object; measured in Newtons (N); varies with gravitational field strength; measured using a scale (measures force). Weight = mass × gravitational acceleration. On Earth: g = 9.8 m/s². Weight of 1 kg on Earth ≈ 9.8 N. Common bathroom scale measures weight (in pounds, which is a force unit), but displays it as if it were mass — works because Earth's gravity is constant on the surface. On other bodies: Moon's gravity ≈ 1/6 Earth's, so a 60 kg person weighs ~98 N on Moon (vs ~588 N on Earth) but still has 60 kg of mass. In free fall or orbit: 'weightless' — gravity acts but no normal force; mass unchanged. The pound is sometimes used as a mass unit (avoirdupois) and sometimes as a force unit (pound-force) — confusion arises in everyday language. SI strictly: kilogram = mass; Newton = force; pound-mass and pound-force are separate.