PEX (cross-linked PolyEthylene) is a type of pipe commonly used in household plumbing. It is quickly becoming the most popular piping for supply lines, although PVC pipes are still the most common for drainage systems.
PEX pipes have the advantage of being plastic (cheaper then metals such as copper), durable, resistant to chemical change, able to withstand temperatures from below freezing up to about 180 degrees Fahrenheit, being very easy to work with, and being flexible. The list of benefits attributed to PEX goes on; by being flexible it requires fewer joints, meaning fewer leaks, and it has a dampening effect on water hammer.
PEX was developed in the 1960s as a variant on polyethylene (PE) pipes. (PE pipes are a type of flexible plastic pipe that is still used for cold water supply lines in circumstances where freezing is unlikely.) There are various methods of producing PEX pipe, but all involve inducing a chemical reaction in the polyethylene to create longer chains of fibers within the plastic ('cross-linking' the molecules together). Although it has been used in Europe since its invention, many American building codes only started allowing it in the 1980s. It is now accepted in most areas of the developed world.
PEX has it's own system of connections; there is obviously no soldering, and the solvent glues used on PVC pipes will damage the PE plastic. PEX uses a combination of barbed fittings inserted into the pipe and compression rings to hold the joint tight. Because the pipe is flexible few joints are needed saving on both labor and money.
It is common, although not necessary, to install a copper manifold to provide a 'junction box' for PEX systems. Rather than a series of 'T' joints spread throughout the system, as you would find in traditional copper plumbing, there is one central strip of 'T' joints from which all out-going pipes flow. This manifold is located at an easily accessible location, and commonly has shut-off valves for each individual out-going pipe. This provides a simple way to isolate a single fixture in case of leaks, reduces labor, and keeps your plumbing simple (it can often be hard to predict where a given joint is to be found in a traditional plumbing system). This does require more pipe than traditional systems, but it is generally deemed worth the slight increase in cost.
There are some downsides to using PEX: it should not be exposed to ultraviolet radiation (e.g. sunlight) over long periods of time; it is easier to puncture PEX than copper or PVC plumbing (although short of accidentally driving a nail into it you aren't like to puncture it); PEX should not be joined directly to other forms of plastic pipe, particularly using a plastic solvent (they can be joined to metal pipes easily). While PEX pipes will not break if they freeze, you cannot use an intense heat source or electrical current to defrost them as you would with copper pipes.
Because drainage systems require a lot of very specific turns (U-bends) and angles, because they require larger diameter pipes, and because some nasty chemicals may get poured down the drain, PEX is not used in these applications. The cheap, rigid, and durable PVC is preferred in most home installations.
PEX is generally pronounced as "peks". You will often hear it referred to as "PEX tubing", although as it gains popularity it is becoming known as "PEX plumbing", or most often, simply "PEX".