TexasMike is correct about the PADI Master Scuba Diver stuff, rescue + 5 specialties. The NAUI MSD course is a couple fewer dives (8 required), no actual specialties done, but the academic content is equivalent to that of divemaster. The NAUI DM course requires Master SD or equivalent, rescue, 25 dives, and NAUI AI water skills ability. PADI DM requires 20 dives (60 to exit course), rescue, advanced or equiv.
Netdoc-- caught my mistake about the IDC, AI OR DM can go. What I meant to say is that you can't become a PADI AI without another leadership rating (any agency). PADI AI is a very useful rating, worth checking out if you don't want to be an instructor. DM's are not required to carry insurance but it's a good idea.
My OPINION is that a crossover from NAUI to PADI is easier than the reverse. Very few candidates from my IE would have passed a NAUI ITC, IMHO. But that's all I'm going to say about that!
Basically NAUI and PADI offer similar (or similarly named) courses all the way through instructor, the difference being that AI and DM's places are reversed in the progression.
Rick-- All the NAUI swim tests are timed still. PADI has a points system instead, faster you go, more points you get with a minimum needed to pass. Also, no bailouts, ditch and dons, scuba or skin, in PADI. I say bring back the torture!
Neil