What is Heat engine ? It's Classification & Difference between 2-stroke engine and 4-stroke engine :

Heat Engine: The machine which converts heat produced by burning of fuel into useful work is known as heat engine. In other words, heat engine is basically a equipment which generates thermal energy and transforms the same into mechanical energy form.
 
There are different types of classification of heat engines based on various criterias.
There are two types of classification based on working cycle. The same have been mentioned in the table below.

In 2-stroke engine, the whole sequence of events i.e. suction, compression, power and exhaust are completed in two strokes of piston i.e. one revolution of crankshaft.
In 4-stroke engine, all the four events (suction, compression, power and exhaust) take place inside the engine cylinder. The 4 events are completed in 4 strokes of the piston i.e. two revolutions of the crankshaft.

The figure-1 depicts 2 stroke engine (petrol type) and 4 stroke engine (petrol/diesel type). In 4 stroke engine all the four events viz. suction, compression, power and exhaust occur in four different strokes of piston. In 2 stroke engine, suction and compression occur in upward stroke of the piston while power and exhaust occur in downward stroke of the piston.