It's Simple: One moving part, a filter, and and a bunch of O-Ring seals.
It's an elegant design, really. Not bad at all -- I have 3 of the Moen cartridge units in my house. I'm guessing there were 4 (Kitchen Sink too), but Dan broke the faucet right when I was buying the house and he had a plumber replace it with a cheap ceramic unit. I know that all 3 are at least 18 years old .. and looking at the corrosion and feel I'd guess they are more like 30 years old.
In the 18 years I've owned my house, I've had many problems with the Kohler inserts on my two other bathroom sinks. I've replaced the inserts and some extra parts 3 times in 18 years ... and spent probably as much on them as I did on a Moen cartridge. They also have a design flaw -- once they start leaking, the non-replaceable seat of the valve body can be eroded by water flow, and then they leak forever. By the way, I really like and respect Kohler products, I'm not raining on their parade -- just giving my experience!
The Moen cartridges, in contrast, never malfunctioned or leaked for a period several times as long. They are always easy to operate.
All this is done with 3 components and O-Rings:
The valve body has ports for the hot and cold water pipes near the rear of the cylindrical tube that contains the cartridge. There is an output port in the middle of the valve body which connects to the faucet/bath/shower. Inside it is just a hollow tube, with slots at the front for the retaining clip that restrains the cartridge.
The cartridge has an outer shell. There are matching hot and cold water ports near the rear of the shell. Two O-Rings, on each side of the ports, prevent hot and cold water from flowing along the outside of the shell in either direction front or back.
A diagonal O-Ring on the outside of the cartridge between the two other O-Rings separates the hot and cold water ports from each other so they don't intermix outside the cartridge.
There are output ports on the shell to match the output ports in the valve body. A third conventional O-Ring near the front of the body keeps the output flow from escaping out of the core of the valve body past the edge of the cartridge.
The retaining ring simply holds the cartridge in place, so that it is not pushed out of the body by the force of the water pressure.... or the operator pulling on the stem :-)
The valve stem is the operating component of the valve cartridge.
The stem is hollow inside, and has multiple inner-mounted O-Ring seals near the front (by the stem where the valve handle attaches) to keep water from flowing out the valve between the cartridge shell and the stem. There is also a retaining ring which prevents the stem from being pulled out of the cartridge (or pushed out by water pressure). These O-Rings also provide friction to hold the stem at a given flow setting.
The inside rear of the stem (where all the ports are) is filled with a fine (finer than window-screen) filter screen. This filter keeps incoming particulate matter out of the valve cartridge. The particles could damage the O-Ring seals or block up the valve.
Thinking about it .. this could also help make water flow turbulent, so that the hot and cold flows mix better in the body of the valve.
When the valve is fully closed, the stem is pushed fully into the cylinder. At that point, the body of the stem blocks both the hot and cold water ports to prevent water from flowing. It is also why water pressure can't push the valve to on -- the water is pushing sideways on the stem, not along it's length.
There is a large port cut into the valve stem near the rear. This port is somewhat symmetrically shaped like a rectangle, but one side has an "ear" going off -- that's the cold water side -- the valve will always flow a little bit of cold water at all time (when open).
When the valve is pulled open the above port starts engaging the hot and cold water ports in the barrel. The more you pull the stem out, more and more of the port is exposed the the ports in the valve body, and more water flows. This controls the volume of water coming out of the valve.
When you turn the stem side to side, it moves the port more towards the HOT or COLD water ports, to make the temperature hotter or colder. This controls the temperature of the water. That little channel cut toward the cold water side always provides a little bit of cold water to help against scalding.
It may also be a warning device -- if you always have a trickle of cold water coming out of the valve -- it means that the stem isn't going fully closed, and you should think about replacing the cartridge.
The water coming in the port on the stem moves forward in the stem to the elongated output ports on the stem. Those ports match the output ports in the barrel and the valve body. Voila -- the mixed water comes out the faucet!
The rearmost of the stem is open. When the valve first starts flowing water, the incoming water escapes out the back of he cartridge (which is sealed to the valve body by O-rings. If that space wasn't "vented" like this, the valve would always want to shut ... or if open, would always want to stay open. Be making the stem hollow, it relieves pressure on both sides. It also provides a drainage port for the rear O-ring seal on the cartridge. That way, if water leaks past the rear O-ring, it won't want to push the stem forward to turn it on by itself.
A O Ring is a round rubber gasket that is used to seal the gap between two cylindrical surfaces. The O-ring is installed into a partial circular groove, which is cut into one of the cylindrical surfaces. which is usually on the inner portion of two mating cylinders.
When no pressure is applied to the joint, the O-Ring naturally provides a positive seal. The O-ring is slightly larger than the distance between the bottom of the groove and the mating surface. When the O-Ring is installed, it must be lubricated, so it can be slid, without damage, into the mating cylinder.
When pressure is applied to one side of the O-Ring, it the ring deforms. The pressure pushes the ring toward one edge of the groove. When the ring reaches the groove edge it deforms to push harder against the cylindrical wall on the other side. In other words, the pressure applied to the O-Ring causes the O-ring to act as a wedge that seals even tighter against the other (non-grooved) surface. The cool thing about O-Rings is that the higher the pressure applied -- the tighter the o-ring seals! Very cool!
Systems using an O-Ring need to have filtered operating fluid. If particles are in the fluid they can cut into the O-ring, preventing it from sealing fully. When water is the operating fluid, it helps to have softened water. This helps prevent the formation of calcium deposits on the cylinder walls, which will also slice through the O-Ring.
What happens to the O-Rings? When they start getting cut, they will start shedding rubber pieces into the cylinder. These might escape out with the normal water flow, or they might jam in the valve ports, or prevent operation of the valve.
If you have a bunch of Moen faucets, keep a cartridge on-hand, and maybe extractor tool. Normal maintenance is a snap, and if things get tough, there are ways to deal with a Stuck Cartridge.
If you notice that the faucet is always warm, or that when you turn on the cold water it is warm for a while... it means that water is leaking past the diagonal O-ring separating the hot and cold water ports.
This leak is due the hot water being at a slightly higher pressure than the cold water, due to the boiler effect of a hot water heater. Why hot-water systems don't have an expansion tank to maintain pressure against this effect ... I don't know.
This is why you don't have cold warm water... though you wouldn't notice it so much since the hot water cools down in the pipes anyway. One nice effect of this leak is that your bathroom hot water is always instant-on, without having to install a hot-water recirculation system.
What it definitely means for your valve is that the O-rings are starting to go in your valve cartridge. If you read How O-Rings Work, you'd see that they should normally not leak.
If you find your cold water is warm .. time to pick up a replacement valve cartridge!
Two things could have happened here, and both mean you need to get a new cartridge soon. Just to make things clear ... the water being off didn't destroy your faucet ... it was already wearing out. It just helped make its problems worse!
Get a cartridge and replace the old one. You should have some time, but you want to replace the damage before it becomes catastrophic.
If the valve never fully shuts off the water flow, leaving a very slight stream which you can control in temperature by valve position ...
Debris has built up so that the valve stem can no longer bottom out against the back of the cartridge to seal off the hot and cold water ports. This could be due to particulate matter, water deposits, or by shredded O-Ring components.
Time to to get a new cartridge and replace it ASAP.
But you managed to get it turned off.
DO NOT TURN IT ON AGAIN -- Better yet, turn the water supply to the faucet off.
Don't even think of turning it on again to test it.
Your Cartridge Has Failed
Replace it NOW
It was difficult to shut off because part of an broken O-Ring or other seal is jammed in a port. Through twisting, turning, and diligent effort you managed to move the stem to its shut position, where water pressure can no longer force the stem open.
You want to shut off the water in case someone else turns it on... because then it will be impossible to shut off -- the valve is broken!
Get a cartridge and replace the old one.
Your faucet has failed.
You can no longer turn it off because the internal mechanisms are broken or jammed.
You must turn off the water supply to the faucet, or the main water valve if there is no faucet valve -- which is typical on bath and shower faucets.
Get a cartridge and replace the old one.