If you have read the HSW article on car engines and the diesel engine page, then you are familiar with the two types of engines found in nearly every car and truck on the road today. Both gasoline and diesel automotive engines are classified as four-stroke reciprocating internal combustion engines.
There is a third class of engines, known as two-stroke engines, that are commonly found in lower-power applications. You will typically find two-stroke engines in things like:
Lawn and garden equipment like chain saws, leaf blowers, trimmers, etc.
Smaller motorcycle engines used on dirt bikes
Small outboard motors
Radio-controlled model planes
You find two-stroke engines used in these applications because two-stroke engines have two important advantages over four-stroke engines:
Two-stroke engines do not have valves, which simplifies their construction.
Two-stroke engines fire once every revolution while four-stroke engines fire one every other revolution, giving two-stroke engines a significant power boost.
These two advantages make two-stroke engines lighter, simpler and less expensive to manufacture. They also have the potential to pack about twice the power into the same space because there are twice as many power strokes per revolution. The combination gives two-stroke engines a great power-to-weight ratio.