Plasma is called the fourth state of matter because three other states of matter exist, are more commonly known and are often referred to as "the three states of matter." Plasmas are not directly encountered by most people in everyday experience. They exist in sealed enclosures such as neon signs, in industrial settings where plasma torches are used to cut metal, or at extremely high temperatures like the Sun's interior.
A plasma is created when a source of energy strips atoms of their electrons so that positively charged nuclei and negatively charged electrons coexist. Sources of energy that create plasmas include natural sources such as stars, and man-made sources such as lasers or focused microwave generators. Relatively low energy sources are often used to create plasmas, such as when a current is applied to a gas in a neon sign or fluorescent light.
Once the electrons have been stripped from the nuclei in a gas, the nuclei are free to fuse together if sufficient energy and pressure are applied. In the plasma of a star's interior, hydrogen nuclei stripped of electrons fuse together to create helium atoms, a process that converts some of the matter to energy such as heat and light that the Sun provides to Earth. A thermonuclear bomb functions in the same way, with a fission bomb providing the heat necessary to strip the electrons from hydrogen atoms, fuse the nuclei and convert some of the matter into a tremendous burst of energy.