A tee is used to make right angle connections between three pipes. The Tee is so-named because it is shaped like the letter T. The only measurement that matters is the overall diameter of the pipes. To select the proper size Tee, make sure that the outside diameter of the pipes is the same diameter as the Tee openings.
A coupling is designed to join, or couple, two pieces of PVC pipes together. The coupling will have the same opening on both sides and will only accommodate two pieces of pipe. You can use a coupling to join different pipe sizes by inserting the proper size bushings. Joints must be secured properly, with PVC cement, after joining the sections.
Adapters come in two types, male adapters and female adapter, and are designed to couple threaded pipe connections to a piece of "slip" PVC pipe. The adapter will have a threaded interior if it's a female or a threaded exterior if it's a male adapter. The other opening will be completely smooth in order to accommodate the "slip" section of pipe.
Bushings are typically used to change the pipe size of a fitting. Bushings come in slip by slip, slip by thread and thread by thread. Bushings come in many different sizes.
A cap fits over the end of a slip PVC pipe. The cap stops all water flow through the end of the pipe and can either be threaded or smooth. Threaded versions don't require cement and can often be secured with Teflon tape or pipe compund; however, smooth versions require PVC cement in order to remain in place.
Crosses are designed to join four pieces of pipe in a single right angle. Shaped like a "plus sign" the cross fitting is essential in many plumbing applications where water must be diverted from a single source and distributed in several different directions. Typically, crosses come only slip connections and, must be secured with PVC cement.