For master diver the specialties required are deep and uw nav. Nitrox counts as do other more useless specialities such as boat diver, fish ID, project aware, etc. If you go for it get some useful ones like search and recovery, night/low vis, drysuit if applicable, wreck, peak buoyancy, etc. Ones that will actually help your skills. The others can be done later when you have money to throw away.
As was said DiveMaster is the first pro level. Some of us refer to it fondly as indentured servitude depending on where you are or who you do the course with. You'll focus alot on theory, teaching methods, organization, and marketing. Your skills will be tested and will need to be demonstration quality. You'll get alot of time in the water and also schlepping gear, cleaning it, putting it away, perhaps "volunteering" at the shop, etc. If you don't plan on ever teaching or trying to work as a divemaster I'd skip it. The theory you can get by reading,the organizational skills by working at most any job, and the marketing is a big waste of time as far as your diving goes.
Actually Deep & Nav are not required for MSD. They are required dives for AOW though.