from http://encyclopedia.thefreedictionary.com/Master+Chief+Petty+Officer
Senior Chief Petty Officer is the eighth enlisted rank in the U.S. Navy, just above Chief Petty Officer, and is a non-commissioned officer. They are referred to as Senior Chief in most circumstances.
Advancement to Senior Chief Petty Officer is similar to that of Chief Petty Officer. It carries requirements of time in service, superior evaluation scores, specialty examinations, and peer review. A Chief Petty Officer can only advance if the existing senior chiefs approve.
As with Chief Petty Officers, a Senior Chief Petty Officer takes on more administrative duties. Their uniform changes to reflect this change of duty; becoming similar to that of an officer, albeit with different insignia than an officer.
Master Chief Petty Officer is the ninth, and highest, enlisted rank in the U.S. Navy, just above Senior Chief Petty Officer. They are referred to as Master Chief in most circumstances.
Advancement to Master Chief Petty Officer is similar to that of Chief Petty Officer and Senior Chief Petty Officer. It carries requirements of time in service, superior evaluation scores, specialty examinations, and peer review. A Senior Chief Petty Officer can only advance if the existing master chiefs approve.
A Master Chief Petty Officer is purely administrative and is usually tied closely with the Commanding Officer, except possibly when the command is very large (e.g. an aircraft carrier). Their uniform reflects this administrative duty; becoming similar to that of an officer, albeit with different insignia than an officer.
-- a Command Master Chief, BTW (as in GI Jane) is a master chief by rank; "Command Master Chief" is a position, not a rank. The CMC is the senior enlisted person in a command, and reports directly to the Commanding Officer (CO). The officer counterpart for a CMC is an Executive Officer (XO), who also reports to the CO.