The difference is how the cylinder is reloaded for the next firing. They both compress the fuel air charge, ignite it, and then allow it to expand to make power. A two stroke reloads while the piston is at the bottom and every revolution of the crank has a compression and expansion stroke. On a four stroke engine, there is a compression and expansion stroke that makes the power, and then the engine coasts for one revoulution during which the cylinder is reloaded for the next firing. It takes two revolutions of the crank to complete a power cycle.

Because you only hear an exhaust eplosion every other revolution on a four stroke engine, they sound like they are reving slower than two stroke engines. Actually, their ability to rev is not that different. As you get to the larger sized engines, it is the four strokes that can outrev the two strokes, this due to the two stroke engine's inability to reload the cylinder completely during the extremely short time of the reloading phase at higher rpms.