Plastic water pipes are made from polymerized vinyl chloride (PVC) and are mostly used outdoors and sometimes for interior water lines. PVC fittings are made to slip onto the ends of the pipes and be glued in place.
The glue partially melts the plastic of the pipe, which then fuses with the fitting to form a watertight seal. Common fittings include 45- and 90-degree elbows, straight couplings, tees and crosses, plugs and reducing bushings, which are used to change the diameter of the pipe.
A bushing is glued into the end of a larger-diameter pipe, then the smaller pipe is inserted into the hole of the bushing and glued in place.
Copper pipe is the type most commonly used for interior water lines, because it's corrosion resistant and relatively easy to cut. Copper fittings are soldered onto the ends of the pipe in a process called sweating.
Elbows, couplings, tees, crosses and plugs are also commonly used in copper, but changes in pipe diameter are accomplished by using bell reducers, which are the obverse of bushings. Bell reducers have inlets of different diameters and are sweated onto the ends of the pipes they connect.
Galvanized steel was used at one time for most interior water lines, but because it's subject to corrosion, it has been gradually replaced by copper. The ends of galvanized pipes are threaded, and fittings are screwed on.
The variety of fittings is the same as for copper and PVC except for unions, which are additional fittings that are essential when working with threaded pipe. A union has two parts joined by a common nut.
Each part screws onto the end of a pipe, and the pipes are then joined by tightening the nut of the union.
There are many adaptors that allow plumbers to switch from one kind of pipe to another. Male and female PVC adaptors have a slip connection on one end and a threaded connection on the other, and plumbers use these to connect plastic pipes to metal.
Adaptors also exist for use with copper pipe, with a sweat connection on one end and a threaded connection on the other. These are often used for connecting valves or flexible supply lines to copper pipes, such as when installing faucets or toilets.
Plumbers also sometimes use compression couplings to connect faucet supply lines to copper pipes. These can be tightened onto the end of a copper pipe without needing to be threaded.