In What Context? V-Twins? Thumpers? 2-4-6 cylinders? Because you are new I would recommend something between 350cc-500cc (Cubic Centimeters), basically something you can ride safely one the freeways. I HIGHLY recommend that you visit some dealerships who can fit you with the right bike for you needs and area.
The engine size which is commonly referred to as "cc" or cubic centimeters is the basic difference. The higher the number does not mean that you will receive more horsepower. You can have a 250cc motorcycle that will produce 20hp or 60hp depending on the mechanical factors.
The main differances are "air cooled" and "water cooled. " The other differances are cylinder placement and amount of cylinders. The types are "V-Twin, inline, single, V-four.
the basic two types of engines are two stroke engine and four stroke engine. in two stroke engine the combustion and display of piston takes place in two strokes. in four stroke engine,this action takes four strokes. four stroke engine is more fuel efficient than the two stroke engine.