Most modern aerosols use a liquefied gas as the propellant. The gas has been turned into a liquid by putting it under pressure. Inside the can, its vapour produces enough pressure to force the product out. As the product level drops, more propellant evaporates to maintain a constant pressure in the space above the product.
Most liquefied gas propellants are mixtures of simple hydrocarbons such as butane and propane. The drawback is that they are flammable.