The head holds the center diamond in position. It could either be a bezel or have prongs. It may or may not have the same metal as in the mounting. If you go with a different color, it will create an interesting contrast.
Prongs are the support structures used to hold a gemstone in its mounting. It usually comes in groups of six or four and is a narrow structure. Clearly, a six-prong structure offers more support than a four-prong one. However, the latter can display the diamond in an impressive way.