Matter is made up of particles. There are three common states of matter: solid, liquid, and gas. These substances look different macroscopically and the particles behave different microscopically. Solids have a definite volume and the particles are in a definite shape. If you could view the particles in a solid microscopically, you would see them vibrating in place. Liquids have a definite volume, but the particles are in a indefinite shape. Microscopically, the particles in a liquid can flow. Gases have an indefinite volume and indefinite shape; both can change.
See a model of three states of matter; solid, liquid, and gas.
Source: kosasihiskandarsjah on Jan 10, 2008
Learn about the different states of matter and the microscopic and macroscopic differences.
Source: Kathryn Kennedy