Mass refers to the amount of matter an object contains. It is a measure of an object's inertia, or its resistance to changes in motion.
Weight: Weight is the force exerted on an object due to gravity. It depends on both mass and the acceleration due to gravity.
Inertia is the tendency of an object to resist changes in its state of motion. It is directly related to mass.
Density: Density is a measure of how much mass is contained within a given volume. It can be calculated by dividing mass by volume.