How do you know plasma is real if you can't see it?
This might be a surprise, but you can see plasma. Or, technically, you can see the energy (light) given off by a plasma. The sun and all stars are composed of plasma. This strange "4th state of matter" is actually the most common form of matter in the universe. A plasma is a collection of atoms (think of a container filled with a gas) which has absorbed enough energy to cause the electrons to become separated from their nuclei. These electrically charged particles are called ions. When this happens, the plasma no longer acts like a gas. This "charged particle soup" now has electrical properties, and creates a magnetic field (because of the electrical field). The excited charged particles radiate light and other forms of electromagnetic radiation.
So what can cause a container of gas to act this way? It takes energy to ionize the gas and create the plasma. We can make it happen on Earth in neon lamps and other devices by using electrical energy. In a star, it works something like this. Large clouds of gas in space are thought to have "condensed" into more dense blobs due to gravitational attraction within the gas. As the blobs grow more dense, the gravitational pull is stronger. Eventually, the atoms of gas are forced into such tight spaces that they can't exist as atoms, and become ionized (the electrons are stripped away), forming a plasma. It doesn't stop there in a star. All this motion causes friction and heating. The heat helps ionize more atoms, causing more plasma to form. Deep in the core of this hot blob of plasma the nuclei of the atoms are squeezed more tightly together by gravity. Then at some point, those nuclei (protons and neutrons) can join together in the process called nuclear fusion. When fusion takes place, it releases large amounts of energy. This energy keeps heating the plasma, giving rise to more fusion, etc. So the sun is a great "plasma/fusion furnace."
For more information on plasma and fusion, check out this website.
Keith Welch, Radialogical Controls Group (Other answers by Keith Welch)