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Due to this question I now have a fairly well-rounded understanding of what a head gasket is. I assumed that sometimes, when people would talk about "heads" that this was just an abbreviation of another term "head gaskets". Apparently, this is not the case!
What exactly are heads?
Clarification: Car heads, not human ones...
tl dr: Cylinder heads (commonly just called "heads") are the big piece of metal which caps and seals the end of the cylinder bore.
There are three basic types of cylinder heads:
Flathead Cylinder Head - These cylinder heads were used on older engines such as the flat-head Ford engines (like seen below). The oval protrusion at the top of the cylinder head is for coolant flow. Coolant would flow up from the block and through passages in the head (between bolt holes) and out through this oval port at the top.
Overhead Valve Head (OHV) - This type of cylinder head has been used extensively from the 1950s and are still used today. These heads have the valves located within them instead of in the block like the flathead. The valves are actuated indirectly from the cam through a valve train, which usually consists of lifters (tappets), push rods, and rocker arms. An OHV head usually looks something like these Chevrolet heads below. In the picture, the flat surface of the head (right side of the image) is the portion which faces the engine block. The left side is where you see the valve tips, valve springs, retainers, and keepers. On top of these is where you would see the rocker arms if they were installed.
Overhead Cam Head (OHC) - This third type of head usually has the camshaft co-located with the cam. It is called "over head", because the cam actually resides over the top of the head. The OHC can have a single (SOHC) or dual (DOHC) configuration. (You can learn more about the differences between a SOHC and DOHC in this SE post). Here are some images showing OHC heads:
In this image, it is a little hard to see, but there is one camshaft (you can see the single gear to the left of the head) which extends the length of the head. This single cam shaft actuates both the intake and exhaust valves which are in the head.
In this image of a DOHC, you can see the two cam shafts (find two gears to the right of the head in the image). with DOHC, the valves can be actuated directly (as in this image) or with the use of rocker arms.
First let's explain the Flathead Cylinder head a little. Like I said before, it is basically just a flat chunk of metal which seals the top of the cylinder. In the image above, the top portion of the image (as I stated) is the flat part. The indentations in the head (the almost heart shaped portion) is the combustion chamber. This is where the air fuel mixture has room to burn when the piston is at top dead center (TDC) during ignition, between the compression and power cycles of an Otto cycle engine.
To better understand what makes up most other cylinder heads, let's go over some of the basics (Note: some of these things were mentioned above, but I'll explain better as to what they are here):
Bare Cylinder Head: This is the main building block of the entire structure. All other components we'll talk about are attached to the bare head in some form or fasion.
Valve Spring Retainer: The valve spring retainer keeps the spring located on the spring. The retainer is located on the top end of the spring (away from the head). It holds the spring in place while simultaneously attaches to the valve via retainer locks (see below). Here is a representative image of a retainer:
Retainer Lock: The retainer lock (or key) is wedged between the valve tip and the retainer. An interference fit occurs because of the spring pressure pushing up on the lock, the angle of the locks in relationship to the straight stem of the valve, and the small tab which fits into the valve. Here is a representative image of a retainer lock:
This is one half of a pair of locks. You can also see the angle which is created, along with the small tab which connects the lock to the valve stem.
Valve Seal - The valve seal is located around the stem of the valve at the top of the valve guide (see below) underneath the valve spring. It provides a seal so as to prevent oil from leaking (or getting pumped) into the intake and exhaust tracts. Here is a representative image of a valve seal:
In the image, you can see two valves (at bottom right and to the left). The valve seal is the spring which encircles the valve, the polymer piece underneath of it, and the metal which is below that.
Valve Guide: The valve guide does exactly what the name implies - it guides the valve. The valve guide is a separate piece of metal usually made out of cast iron or bronze. The guide is separate from the head so as it can be replaced if needed when a head is rebuilt. It is driven into the head and provides a way to keep the valve specifically located while still allowing the valve to move up and down during the open/close cycle. There are many different types and different applications for each. Here is a representative image of different valve guides:
Valve Seat: The valve seat is a hardened material which is hammered into the head, then cut at specific angles to, along with the valve, create a seal which keeps the gasses during the combustion process locked into the combustion chamber. Valve seats are usually made of some type of iron alloy as this tends to stand up to the rigors of an opening/closing valve over its lifetime. You can find seats in cast iron heads (most of the time), but you'll always find valve seats in aluminum heads. Without these, the valve would move the aluminum out of the way and would soon be damaged. Here is a representative image of a valve seat:
In the image, I've added two yellow ovals. The valve seat is located between the two ovals. The seat is the portion which is discolored and looks to be machined at an angle. When the seats are place, a place is machined into the head where the seat will later be driven into it. Then the seat is cut at specific angles so the valve will make perfect contact with the seat and create a positive seal. You can also see in the image, at the center of the circles, is the valve guide peaking out at you.
Spring Seat: Depending on the head manufacturer and the head design, there may also be a spring seat, which can protect the cylinder head from being galled by the spring. As you may have guessed, the spring seat straddles the valve guide and is placed between the spring and the head. You'll usually see these on aluminum heads. In this area, a manufacturer can also place valve spring shims to set the spring height correctly.
Here is an image of most of the parts described above and how they all fit together (blown apart, of course):
Combustion Chamber: The combustion chamber is where all the action occurs. The shape of this, in addition to the valves and dome (or dish) of the piston, shapes how the flame front travels after air/fuel ignition occurs. A combustion chamber can be of many different sizes and shapes. It all depends on what is needed for the particular engine. Below is a representative image of a big block Chevy (BBC) closed port combustion chamber. In the image, you see the face of the intake (larger valve) and exhaust (smaller valve) valves. Just to the right side of the valves (at 4 O'Clock to the valves) is a threaded hole. This is where the spark plug protrudes from.
Here is a very basic OHV head. You can see the different parts of the head annotated for better understanding. All head with valves in them work fairly much the same way, but can be arranged in different ways as you've seen in the above images. Here is another image showing the cross section of how the valves, intake runner and exhaust runner may be situated within the head:
Intake/Exhaust Ports: The intake and exhaust ports are the part of the head which allows the air/fuel mixture to enter the cylinder and the exhaust gasses to leave the cylinder. The size, shape, and texture of these can help or hinder how the air flow occurs. With greater flow comes greater power and torque. Here is representative image of a cutaway view of a port:
Extra Bits & Pieces: During the casting of heads, coolant passages are built into them. This allows the head to bleed off excess heat from the combustion process.
A 4th type of head, the jughead. In old Buicks before the 1920's, it was overhead valve but the head was not removable, the valves were removable because they were in cages but the head was not. To take the head off you had to go down lower and remove the whole upper half of the engine including the jugs. So it was called a jughead engine.
"Head" in general refers to an end or closure of a pressure containing part ,such as an engine cylinder head, heads of a pressure vessel , heads of a barrel, etc. Nothing specifically to do with gaskets. And a second use of "head" for pressure , as a "head of steam". The definition using "head" to mean an end has been used generally as ;head of a bolt or pin, etc.
