A tee has 3 solvent welded (or threaded in the case of steel) openings. Steel tees also come in weld-in types, but these are mainly for industrial use. Flanged or grooved tee fittings are used for huge pipes.
These are arranged in a planar arrangement, each skewed 90* from the last. This looks a bit like the letter "T", which is where the fitting derives it's name.
A more advanced use of tee fittings is as the basis for both barrel- and chamber-sealing piston valves. The valve itself is constructed inside the tee, and the outlets are connected to the chamber, barrel and pilot valve.