A top every engine block sits a mechanical assembly that serves as traffic cop letting fresh air and fuel enter the engine cylinders and exhaust gases exit. This device is the cylinder head, and it opens/closes to let these gases come and go as needed to keep the engine running.
The cylinder head consists of a casting, typically made from aluminum or iron that bolts to the top of the engine block, closing off the tops of the cylinders in which the pistons move. The head can be for a single cylinder or for multiple cylinders. Each head is attached to the block using studs, bolts, or a combination of the two. Between the two is a high-temp gasket used to seal the piston bores and water passages. On a water cooled engine, water passages incorporated in the head are used to route coolant to/from the block and/or intake manifold.
 The intake ports on the side...  The intake ports on the side of the head are shaped and sized to accommodate the input charge flow. A smooth flowing transition is desirable and will optimize efficiency and power, but production heads are built within the limits of raw castings. Modified "ported" heads have been carefully shaped and smoothed manually using various die grinder tools to increase performance. This process is not entirely intuitive and the art of porting is a valued service. |  The exhaust ports needn't...  The exhaust ports needn't be as large as the intake ports to handle a given engine flow. Similar to the intake ports, these can be reshaped and smoothed to gain the most efficiency from the engine. With concern to intake and exhaust ports, bigger is not always better. Low end torque is often gained from maintaining high port flow velocity at low engine speeds. Too large a port slows the incoming charge and can somewhat negate this characteristic. For wide open throttle and power, larger ports may be better. |  Looking from the bottom side...  Looking from the bottom side at a portion of the head we can see the machined surface that mates with the top of the engine block and caps the piston cylinder. In this case, two valves and a spark plug tip reside in each combustion chamber in this head. |
Intake ports or passages on the side of the head mate to the intake manifold. This is where the incoming fuel mixture enters the head. When an intake valve in the head is opened, the incoming fuel charge flows from the intake port through the valve opening in the combustion chamber. The intake valve closes so cylinder compression and firing can take place. Once the spark condition has occurred, an exhaust valve opens, allowing the spent gases to exit the head into the exhaust manifold.
For many decades heads used only two valves per cylinder: one intake and one exhaust. With the push for higher performance and efficiency, designers found that using two smaller valves to the one larger valve increases flow into and out of the head. As such, heads have been designed and are in production with two, three, four and five valves per cylinder.
Each valve is shaped like a thin steel rod with a tapered circular disc at its end. The rod portion slides up and down in a tubular guide in the head as the valve opens and closes. The circular disc sits on a valve seat when closed to seal the intake or exhaust opening and then lifts off the seat as needed to open the valve.
 The intake valve (left) is...  The intake valve (left) is always larger than the exhaust valve (right) to allow for greater intake flow as the intake pressure is less than the exhaust pressure. Note the keeper grooves on the end of each stem. |  Engine valves are typically...  Engine valves are typically made from steel alloys. Most heads use two valves per cylinder, but some high performance designs use four (two intake, two exhaust) valves to utilize more of the surface area in the combustion chamber as a flow area. Since a combustion engine is only as powerful as its ability to flow a gaseous mixture in and exhaust out, increased flow means greater power potential out of a given engine. |  Here is another view of the...  Here is another view of the combustion chambers, but with the valves removed. The odd shaped passages towards the outside of the head surface are water passages for cooling purposes. Of interesting note, the exhaust valve is typically the hottest component in an engine and relies on contact with the valve seat (closed) for much of its cooling. |
A rocker arm is often used to push on the end of the valve stem to open it. The rocker gets mechanical input from an elliptic lobe on a rotating camshaft so that the lobe pushes and causes the valve to open and close as the engine rotates. A spring on each valve rod keeps the valve closed whenever the rocker is not pushing to open it.
At 6,000 rpm, an engine is asking each valve to open and close 50 times per second. That's a lot of fast mechanical action. Proper, constant lubrication of the valve train is needed to keep the contact surfaces cool and retard wear. If too light a valve spring is used then the valve may not return to its close position quick enough as engine speed increases. This is referred to as "valve float" and prevents complete cylinder sealing for the combustion process. Use of higher rate springs help prevent this problem at high engine speeds, but using overly stiff springs can increase valve train wear.
As a cylinder head ages, there are a number of wear mechanisms that may need to be addressed during a rebuild. The valve face and its mating seat may be worn or pitted causing the sealing juncture to be poor. The valve guide that the valve stem slides in can become loose. As the rocker pushes on the valve stem, there is a slight scuffing motion that loads the stem sideways. Over time, this can cause the valve guide to wear. A worn guide (along with a drying valve stem seal) can start to allow oil from the top of the head to seep down the guide into the valve area. When the engine is started after sitting a while, puffs of blue-white exhaust smoke may signify this aging.
 Each valve stem slides in...  Each valve stem slides in a tubular guide (often bronze or a similar alloy) that is pressed into the head. As these wear over time, oil can start to slowly seep from the valve train area down into the engine cylinders, resulting in some exhaust smoke. The valve guides also serve some duty to conduct heat from the valves into the cylinder head. |  With the valve rapidly opening...  With the valve rapidly opening and closing many times, the casting surface on the head would never survive the wear and tear of this action. So, a hardened valve seat is pressed in the casting for this purpose. This thin ring of material is manufactured from a hard steel alloy. |  The valve face and its mating...  The valve face and its mating seat are ground to allow for tight sealing when they are in contact, but can also be ground to improve flow when the valve opens. A standard valve grind cuts the face with a single angle, while a performance grind will cut the mating surface at multiple angles. |