Crankshaft
The automotive crankshaft converts the liner (up and down) motion of the pistons into the rotational spin that eventually ends up at our wheels. This crankshaft is held in the engine block in a fixed position by the main bearing caps. It has round lobes (main crankshaft journals)
all in a straight line that are fastened to the engine block by these caps. This straight line allows the crankshaft to spin. Other lobes offset or away from this center line are connected to the connecting rods (connecting rod journals) usually with a counter lobe on the opposite end of this crankshaft. As the pistons push down on the rods in each cylinder the crankshaft turns.
The crankshafts rod journals are placed in a certain sequence and this sequence will dictate the engines firing order. The distance from the center of the crankshaft ( the part that rides in the block in a straight line) and the connecting rod journals ( the ones that rotate around this straight line) effects the stroke of the engine.
Automotive crankshafts have a lot of stress placed upon them from the forces pushing them in different directions.
Replaceable bearing inserts are used to absorb these forces and the friction caused by the turning of the crankshaft. There are a lot of reasons crankshaft bearings fail. They will wear prematurely due to causes other than wear. Oil can become contaminated or just left unchanged for to long, heat, pressure, etc. They are know as replaceable items and can be thought of as a way to protect our crankshaft. These bearings are always replaced when rebuilding an engine.
Oil lubricates the crankshaft through holes in the engine block and by splash caused by its spinning action. These oil holes must be lined up with the hole in the bearing. Ignoring this will cause engine failure. There are main bearings connecting the crankshaft to the engine block and connecting rod bearings connecting the crankshaft to the connecting rod.
Another problem inherent to the crankshaft is vibration. The crankshaft actually twists and bends as the different loads are placed upon it. There are different ways we control the vibration caused by these effects.
The engines flywheel is placed at the output or the end connected to the transmission of the crankshaft. These rather large weights absorb a lot of the vibration caused by the automotive crankshaft. 
The torsion or twisting effect caused by the cylinders closest to the front of the engine are absorbed by the torsion damper/harmonic balancer. Counter weights are used to offset the pressure placed on the crankshaft by the forces applied in the opposite direction. Some engines use a counter or balance shaft to absorb vibration.
In the automotive industry while the pistons are considered the heart of the automotive engine the crankshaft is considered the backbone.