At the point where the spark plug fires, fuel and air in the cylinder have been compressed, and when the spark plug fires the mixture ignites.
The resulting forces drives the piston downward.

As the piston moves downward, it is compressing the air/fuel mixture in the crankcase.

As the piston approaches the bottom of its stroke, the exhaust port is uncovered. The pressure in the cylinder drives most of the exhaust gases (but not all) out of cylinder.

As the piston finally bottoms out, the intake port is uncovered. The piston's movement has pressurized the mixture in the crankcase, so it rushes into the cylinder, displacing the remaining exhaust gases and filling the cylinder with a fresh charge of fuel.

Now the momentum in the crankshaft starts driving the piston back toward the spark plug for the compression stroke.

As the air/fuel mixture in the piston is compressed, a vacuum is created in the crankcase. This vacuum opens the reed valve and sucks air/fuel/oil in from the carburetor. Once the piston makes it to the end of the compression stroke, the spark plug fires again to repeat the cycle.

(Two stroke Engine operation
Animated Gif constructed from a flash animation at How Stuff Works.)

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