MWD= Measurement while drilling,
DD= Directional drilling/ driller
MWD provide data while the oil well bore hole is being drilled such as direction the drill bit is going as well as depth, pressure, etc. Measurements are done by instruments that are part of the drill string, hence the term, Measurement While Drilling (MWD)
A directional driller works with the rig's driller to ensure that the well is drilled in the correct direction to hit the oil and/or gas payzones in the earth's formation.
Both who works in the oil field exploration and production (E&P). As to understand all aspects of the work, there is non better than joining an oil and gas service company and getting the hands on experience.
Most DD's come from a MWD background.
Sperry Sun/ Halliburton is one such company. Another will be Baker Hughes' Inteq.
Checkout Rigzone.com, there you can search for other service companies, as well as books that deal with MWD and DD.
Good luck