Matter is anything that occupies space and has mass. It is composed of small particles that are in constant motion and hold together through chemical bonds. The amount of empty space between particles determines the state of matter, with solids having the least empty space and gases having the most. Changing the temperature of matter affects the motion and bonding of its particles, allowing it to change states between solid, liquid, and gas through processes like melting, freezing, evaporation, and condensation.