The cylinder block is a casting generally made out of iron or aluminium and holds the crankshaft, connecting rods, pistons and camshaft (cam-in-block, overhead valves only). The cylinder block has numerous machined surfaces to provide a precision fit to mating parts.
The cylinder block serves as the main structural component of the engine and houses what's commonly referred to as "the bottom end" (crankshaft, rods, pistons). The cylinder block is extremely strong so it can withstand the rigors of engine torque and vibration, while supporting all attached engine accessories and the transmission.
The engine in your car will last for many thousands of miles if driven and cared for properly. The best way to care for the cylinder block is to follow a good maintenance regimen. This includes regular oil and filter changes, engine performance check-ups, and cooling system maintenance. If you notice that engine performance drops off, that the engine is using oil, or observe problems with coolant temperature or oil pressure, it's wise to have these looked into as soon as possible by a qualified professional.