In chemistry one can recognize that the four states of matter are solid, liquid, gas and plasma. The first is rigid, and has a definite shape and volume. The second doesn't have a shape, and assumes the shape of its container, but it has a fixed volume. The third doesn't have either a shape or a fixed volume and assumes the volume and shape of its container. What about the fourth one (plasma)?
Plasma is made up of ionized gas, so molecules have a positive electric charge and valency electrons are totally or partially separated by their nuclei.
Plasma is different from a gas since it has a high temperature and is radiation emitting; think of the Sun and other stars, which are made up of plasma and show both these properties.
Plasma hasn't got a proper volume, like gases; e.g. stars can expand or contract under the opposite effects of gravity and nuclear fusion. For example, this property is important to comprehend the formation of white dwarfs and neutron stars, process caused by high pression due to gravity.
Little trivia: there is also a fifth state of matter, whose name is Bose-Einstein Condensate (BEC).