The piston engine is a metal block containing a series of round chambers or cylinders. These chambers may be arranged in line or in a V; hence, the description of an engine as an inline 4 or 6 or a V-6. The upper part of the engine block is usually an iron or aluminum-alloy casting. The casting forms outer walls around the cylinders with hollow areas in between, through which coolant circulates. The lower block provides a number of rigid mounting points for the crankshaft and its bearings. The lower block is referred to as the crankcase.
The crankshaft is a long, steel shaft mounted at the bottom of the engine and free to turn in its mounts. The mounting points (generally four to seven) and the bearings for the crankshaft are called main bearings. The crankshaft is the shaft which is made to turn through the function of the engine; this motion is then passed into the transmission/transaxle and on to the drive wheels.
Attached to the crankshaft are the connecting rods which run up to the pistons within the cylinders. As the air/fuel mixture explodes within the tightly sealed cylinder, the piston is forced downward. This motion is transferred through the connecting rod to the crankshaft and the shaft turns. As one piston finishes its power stroke, its next upward journey forces the burnt gasses out of the cylinder through the now-open exhaust valve. By the top of the stroke, the exhaust valve has closed and the intake valve has begun to open, allowing the fresh air/fuel charge to be sucked into the cylinder by the downward stroke of the piston. The intake valve closes, the piston once again comes back up and compresses the charge in the closed cylinder. At the top (approximately) of this stroke the spark plug fires, the charge explodes and another power stroke takes place. If you count the piston motions in between power strokes, you'll see why automotive engines are called four-stroke or four-cycle engines.
While one cylinder is performing this cycle, all the others are also contributing; but in different timing. Obviously, all the cylinders cannot fire at once or the power flow would not be steady. As any one cylinder is on its power stroke, another is on its exhaust stroke, another on intake and another on compression. These constant power pulses keep the crank turning; a large round flywheel attached to the end of the crankshaft provides a stable mass to smooth out the rotation.