Solid is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does.
The atoms in a solid are tightly bound to each other, either in a regular geometric lattice (for example crystals like diamonds or crystalline solids, which include metals and ordinary water ice) or irregularly (an amorphous solid such as common window glass).
Liquid is the only state with a definite volume but no fixed shape. A liquid is made up of tiny vibrating particles of matter, such as atoms and molecules, held together by intramolecular bonds. Water is, by far, the most common liquid on Earth.
Like a gas, a liquid is able to flow and take the shape of a container. Unlike a gas, a liquid does not disperse to fill every space of a container.
The density of a liquid is usually close to that of a solid, and much higher than in a gas. Therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, they are both called fluids.
Gas is a compressible fluid. Not only will a gas conform to the shape of its container but it will also expand to fill the container. A pure gas may be made up of individual atoms (e.g. a noble gas or atomic gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or compound molecules made from a variety of atoms (e.g. carbon dioxide). At temperatures below its critical temperature, a gas is also called a vapor, and can be liquefied by compression alone without cooling.
Plasma is ionized gas. A gas is usually converted to a plasma in one of two ways, either from a huge voltage difference between two points, or by exposing it to extremely high temperatures. Like a gas, plasma does not have definite shape or volume. Unlike gases, plasmas are electrically conductive, produce magnetic fields and electric currents, and respond strongly to electromagnetic forces.
Plasmas are by far the most common phase of ordinary matter in the universe, both by mass and by volume.
Artificially produced plasma can be found in plasma displays, neon signs, fluorescent lamps or plasma balls (sometimes called a plasma sphere or plasma globe).
Terrestrial ("terrestrial" refers to things related to land or the planet Earth) plasmas include lightnings, sprites, the polar wind and the polar aurorae.
Space and astrophysical plasmas are the sun and the stars, solar winds, the interplanetary medium (space between planets), the interstellar medium (space between star systems) and the Intergalactic medium (space between galaxies).
