Most people are familiar with three states of matter: solid, liquid and gas. Actually, if you dig into the world of physics, there are several more, but for now we will only add plasma to the list, and we will look at the first three states before talking about plasma.
Things like rocks, wood and ice are solid. Solids stay the same size and shape, no matter what container we put them in.
Things such as alcohol, oil, and water are liquids. They stay the same size, but they change their shape to fit their container.
The most common gas, the air, is actually a mixture of several gases. Gases change their shape to fit their container, just as liquids do. They also change their size to fill their container.
The fourth state of matter is called plasma. Do not confuse this plasma with the plasma in blood. That is something completely different. Plasma as a state of matter is similar to a gas. It changes it's size and shape to fit a container. The difference is that in a plasma, each of the atoms has lost its electrons. These free electrons are moving around between the atoms. For this reason, plasmas are good conductors of electricity. Plasma also gives off light, which makes it easy to see.