Margin is the amount of money that you must keep in your account for each position that you hold. Different futures contracts have different margin requirements which are set by the commodity exchanges and which may fluctuate from time to time.
Initial Margin is the minimum amount of money you must have in your account (at the close of trading) on the first day you establish a new position.
Maintenance Margin is the minimum amount of money you must have in your account after the first day. The amount is always slightly less than the Initial Margin.