Solid
Has a definite shape and a definite volume. This means it can hold its own shape and it takes up a certain amount of space. The particles in a solid move slower and the particles are close together. Particles of a solid vibrate, but do not change position.

Liquid
Has a definite volume, but doesn't have a definite shape. This means it will take the shape of the container that it is in, but it takes up a certain amount of space. The particles move a little faster than those of a solid, but the spacing is similar to that of a solid. Particles of a liquid are free to move around eachother, therefore liquids can flow.

Gas
Has no definite shape and no definite volume. This means that a gas will take the shape of the container that it is in and it will completely fill that container. The particles move very fast and the particle spacing is much greater than that of liquids. Like liquids, the particles of a gas are free to move around eachother, therefore gases can flow.

Plasma
A gaslike mixture of charged (+ and -) particles. Plasma is like a gas, but the particles move so fast that they break apart into positive and negative smaller particles. This causes a lot of energy to be released in the form of heat and/or light.

Plasma is the most common state of matter in the universe. Stars and the sun are all made of plasma.