Gases have unique characteristics compared to liquids and solids. They expand to fill their container and are highly compressible with low densities. To describe a gas, its volume, amount, temperature, and pressure must be specified. The behavior of gases is explained by kinetic molecular theory, which describes gases as particles in constant random motion. Real gases deviate from ideal behavior at high pressures and low temperatures due to intermolecular forces. The van der Waals equation accounts for these non-ideal effects.