Plasma occurs when matter is subject to extremely high temperatures. At low temperatures, atoms lock together to form a rigid structure, such as a crystal. Higher temperatures loosen the atoms' bonds and bring them into a liquid state. At even higher temperatures, the bonds between atoms are completely loosened, making each free-floating--a gas. Extremely high temperatures, such as that of the sun, cause the bonds within the atom to deteriorate. The electrons of the atoms break away from the nucleus, creating a gas that does not consist of individual atoms. This is plasma.
Solids, liquids and gases consist of individual atoms, while plasmas are homogeneous collections of charged particles. Plasma gas is a free-flowing mix of the electrons and nuclei of the broken down atoms.
While the binding of an individual plasma may be uniform, the temperature and density in different plasmas are not. Different elements enter the plasma state at different temperatures. These temperatures are all very high, but still can have a huge difference between elements. Likewise, the number of protons and neutrons in elements' atoms vary greatly, causing different plasmas to have much different densities.
Because of their state as free-flowing electrically charged particles, plasmas have several unique characteristics. The number of protons and electrons in a plasma is even, making it electrically neutral, but because they are in a free-flowing state, electric and magnetic fields have unique effects on plasmas. These fields can influence plasmas over great distances and cause plasmas to form a coherent quality, such as the Earth's ionosphere, which normal gases do not have.