The equation PV = nRT is called the ideal gas law. It states that pressure times volume equals the number of moles of gas molecules times temperature times the ideal gas constant. By convention, pressure is converted into atmospheres (atm), volume into liters (L) and temperature into degrees Kelvin (K).
Chem Guide explains that by rearranging the equation, the units of the ideal gas constant R are L x atm / moles x K, and the value of R is 0.082057 L atm / (mol K). In other unit systems, especially SI, the units of R are Joules / (mol K), and its value is 8.314.
At standard temperature and pressure, or 1 atmosphere and 273 K, 1 mole of gas occupies 22.4 liters. The equation accurately predicts the behavior of actual gases across a range of pressures and temperatures. Under extreme conditions, however, actual gases deviate from ideal behavior. At low temperatures or high pressures, for example, gas molecules do not collide elastically but instead clump together to form liquid droplets.