Cylinder heads are among the most complex components on any engine. Technically a sub-assembly in and of itself, the cylinder head controls every aspect of airflow into and out of the engine. Cylinder heads don't typically go bad like a gallon of milk, but their complex nature ensures plenty of opportunity for failure should something go wrong.
One of the most common failure points for any cylinder head -- particularly those that use multiple intake or exhaust valves -- lay between the valves themselves. High temperatures and pressures in the combustion chamber will cause metal on different parts of the head to expand at different rates, and the thinnest areas will expand and contract more quickly than thicker areas. This is especially true of areas exposed to great thermal differentials -- an area which is far hotter on one end than the other -- as is often the case with the bridge between valves. Cracks also tend to occur in the metal bridge separating the individual chambers, often leading to a blown head gasket.
The entire head can warp or crack under certain circumstances, particularly those related to extreme pressure or thermal differentials. Overheating an engine places extreme stress on the cylinder heads, especially when the heads are aluminum and the engine block is iron. These metals expand at similar rates at normal operating temps, but the rates change drastically as temperatures soar. The hard iron block will always win this fight, forcing the aluminum head to conform to its expansion and splitting it in two. Thermal cracking goes the other way, too; pouring cool water into a hot and coolant-less engine may well split the heads, especially if the engine uses a reverse-flow cooling system which cools the heads before the block.
Head warping is cracking's insidious little brother, sharing many of the same root causes -- primarily overheating -- but manifesting in different ways. Warping happens when the aluminum head gets hotter in one area than in another and the entire head bends or twists a little after cooling down. The engine block will keep the head from simply turning into a pretzel, but iron blocks can impose a whole new set of problems since they'll force the head to re-settle at the iron's expansion size instead of the aluminum's original size. Blown head gaskets will inevitably result, and new gaskets won't solve the problem if you don't take the head to a machine shop to have it milled flat again.
Valve guides are the cylindrical metal pieces that fit in between your valves and head, and they keep the valves from rubbing on the cylinder head. Stock valve guides are usually made of cast iron, which is cheap and fairly durable; aftermarket builders generally prefer bronze valveguides, since bronze is self-lubricating and will generally last for many years of hard use without replacement. Worn valve guides will allow the valve to walk around in its bore, and will allow oil to enter the combustion chamber. Worn valve guides typically manifest first as oil consumption and smoke, then as a miss, loss of power and erratic idle.
