Gases, along with liquids and solids, are one of the states of matter. Gases are usually very low density and compressible (which means it changes volume when pressure increases, as opposed to liquids which are generally incompressible). Gases are compressible because the gas particles have lots of space in between them. Gases are fluids, which means that they flow easily. Gas molecules are arranged randomly and can move freely at high speeds. Most materials will turn into a gas if they get to a high enough temperature. In addition, when we study a contained system of gas, the ideal gas law can be particularly useful.
The ideal gas law gives a fair amount of information about how much energy is in a gas. The energy in a gas can turn turbines and move pistons. Most forms of primary energy use this energy in a gas to get useful work. This includes any heat engines (like fossil fuels, biofuels, geothermal power or nuclear power creating motion or electricity) and wind power.
The University of Colorado has graciously allowed us to use the following Phet simulation. Explore the simulation to see how states of matter change depending on temperature and pressure: