Air Filter: Sits between the air intake and the engine. Filters out debris from the air so it doesn't get into the engine. The air from the air filter goes to the intake manifold.
Alternator: The alternator is a small electrical generator which supplies electricity while the engine is running. It creates electricity from the rotation of the engine. It operates on the principle that spinning a coil inside a magnetic field creates electricity. The alternator is basically just an electric coil suspended in a magnet. As the engine spins it, electricity is created. The alternator is used to charge the battery as necessary, so the battery always maintains a good charge.
Battery: Supplies 12 volts of electricity for when the engine is turned off. (Batteries in some very, very old cars are 6 volts.)
Calipers: What the brake pads are attached to in disc brakes. The calipers straddle a disc attached to the wheel, and when activated, the calipers squeeze that disc, creating friction that stops the car.
Camshafts: The shafts which operate the engine valves. SOHC (Single Over-Head Cam) engines have one camshaft which spins and opens the valves through linkage levers. DOHC (Dual Over-Head Cam) engines have two camshafts which sit over the valves and operate them directly; One camshaft operates the intake valves, the other operates the exhaust valves.
Carburetor: In older cars, a mechanism which regulated fuel flow to the engine. Replaced in newer cars by electronic fuel injection (EFI).
Clutch: In cars with a manual transmission, the clutch is a mechanism to disengage the transmission gears so you can change gears while driving.
Coil: See Ignition Coil.
Connecting Rods: The rods which join the crankshaft to the pistons.
Crankshaft: The shaft attached to the engine pistons, from which the energy created by the engine goes into useful work. The crankshaft is attached to the flywheel, which in turn drives the car through the transmission.
Distributor: The distributor distributes electric power to the spark plugs. It takes in electricity from the ignition coil. Inside the distributor is a spinning piece of metal known as a rotor which touches terminals connected to the spark plugs as it spins. In this way, it sends electricity to each spark plug one at a time when it is that plug's turn to fire.
Engine Block: The most fundamental piece of an engine. The engine block is just that: A big block of metal through which holes are bored, forming the cylinders that the pistons pump through to make power.
Exhaust Manifold: The opposite of the intake manifold. A funnel-shaped metal chute through which the exhaust gases pass after being burned in the engine. In some performance cars, the exhaust manifold is replaced with headers. The exhaust manifold leads into the exhaust pipe.
Flywheel: A wheel attached to the crankshaft which drives the transmission.
Fuel Filter: A filter between the fuel tank and the engine, which helps remove debris from the fuel.
Fuel Injector: The little nozzles which spray fuel into the engine in electronic fuel injection (EFI) systems.
Headers: Tubes leading from the engine through which exhaust gases pass. Headers take the place of an exhaust manifold, and they do exactly the same thing, except that they provide better air flow, because each cylinder of the engine has its own header, instead of having all the cylinders be funneled into one big chute.
Ignition Coil: The ignition coil acts as a transformer. It converts the 12 volts supplied by the battery to a much higher voltage, which is then passed to the spark plugs. This is needed because the sparks from the spark plugs are usually several thousand volts.
Intake Manifold: A metal funnel-shaped chute through which air enters the engine cylinders.
Muffler: A device to reduce the noise made by a car engine, usually situated at the end of the exhaust pipe.
Oil Pan: A bowl in which the oil settles and is stored in while the engine is off. While the engine is running, the crankshaft splashes around in the oil in this pan, thusly lubricating itself.
Pistons: Inside each engine cylinder is a piston which moves back and forth. The pistons each have a rod which is officially known simply as the "connecting rod" which joins them to the crankshaft.
Piston Rings: Metal rings which are wrapped around the circumference of each piston to prevent oil from leaking into the combustion chamber, or fuel leaking out. Piston rings tend to wear down from normal use of the engine, resulting in an engine which burns oil. This is a problem which can only be fixed by rebuilding the engine.
Rotor: The small spinning object in a distributor through which the electricity reaches the spark plugs. Also, the discs on disc brakes are sometimes called rotors.
Spark Plugs: The electrical plugs which create sparks. These sparks, in turn, ignite the fuel/air mixture in the combustion chambers.
Spark Plug Wires: Wires which connect the spark plugs to the distributor.
Starter Motor: A powerful electric motor which turns the engine with great force to get it started. The starter motor gets its power from the battery.
Throttle: A flap which opens and closes to increase or decrease the air flow to the engine. The throttle is connected to the gas pedal. Opening the throttle (pushing the gas pedal) lets more air flow into the engine, making it run faster, and vice-versa.
Throttle Cable: A thin steel cable which is attached to the throttle flap at one end, and the gas pedal at the other. Pushing the gas pedal pulls on the throttle cable, pulling the throttle open.
Timing Belt: A belt with precisely-spaced teeth in it which controls the delicate timing mechanisms of the engine so everything runs in proper sequence. Timing belts can sometimes get stretched or have their teeth worn out, which makes the engine run roughly because it is mis-firing due to incorrect timing.
Transmission: The transmission is sometimes called the "gearbox", and it is indeed a box of gears: It's filled with different gears of varying sizes which are meant to mesh with the engine's output shaft. The transmission sits between the engine and the mechanisms that actually turn the car's wheels. Its purpose is to allow the engine to turn the wheels at different speeds without forcing the engine itself to always speed up to make the wheels go faster. The analogy with the transmission's gears and the gears on a bicycle is entirely appropriate, since they use the same principle.
The most distinguishing characteristic of a transmission (from the driver's perspective, at least) is whether it is an automatic or a "standard" transmission. (I put "standard" in quotation marks because, in reality, a manual transmission, or "stick-shift", is actually much less common than an automatic, at least in North America.) A standard transmission requires the driver to move a gearshift lever that selects what gear the transmission is using; An automatic transmission does this automatically, and so the driver must simply select whether the car should go forward, go backward, or stop.
The second most distinguishing feature of a transmission is how many gears it has. In a standard transmission, this is readily apparent by simply checking the gearshift lever: You can see how many gears it has by looking at the diagram on the top of the lever. An automatic transmission still has these different numbered gears, but they are invisible and transparent to the driver, since the driver does not have to worry about them. Even so, they do exist: When your car starts going faster, but the engine suddenly drops in its rotation speed, your transmission has upshifted to a higher gear. Similarly, when you are slowing down but your engine suddenly jumps to a higher rotation speed, your transmission has downshifted to a lower gear.
Valves: The passages through which air enters and exits the engine cylinders. Each cylinder must have at least two valves, one for the intake and one for the exhaust. Many engines (most engines today, in fact) have more than one valve for each of these purposes, which increases engine performance.