M0: Monetary Base, the actual federal reserve notes (dollar bills) and coins in circulation.
MB: M0 + coins and notes in bank vaults and money deposited at the Federal Reserve (this is the most important measure).
M1: M0 + travelers checks, demand account balances.
M2: M1 + savings account balances, CD’s and money market accounts.
M3: M2 + large deposit accounts and other special Federal Reserve quantities.