real gas
The ideal gas is a useful cartoon, but actual gases — the air you breathe, the steam in a kettle, the carbon dioxide in a fizzy drink — are made of real molecules that do take up room and do tug on one another. A real gas is simply any actual gas, described honestly, with those two facts put back in.
Those two complications pull the gas's behaviour away from PV = nRT. The weak attractions between molecules make the gas a little easier to squeeze than ideal, while the finite size of the molecules makes it harder to squeeze once they are packed close. Which effect wins depends on the pressure and temperature, and the deviation is summarised by the compressibility factor.
Real-gas behaviour matters wherever conditions are extreme: deep gas pipelines, refrigeration, and any process near where a gas would liquefy. The departures from ideal also carry information — measuring them reveals the strength of the forces between molecules, making real gases a window onto intermolecular interactions.
Carbon dioxide under the high pressure inside a fire extinguisher departs sharply from ideal behaviour and can even be made to liquefy — something an ideal gas, by definition, can never do.
Under high pressure a real gas like CO2 strays from ideal and can liquefy.
There is no sharp line between ideal and real gases — every gas is real, and ideal is just the limit it approaches as it becomes hot and dilute. The question is always how large the deviation is, not whether one exists.