Margin is the amount of money set aside in your account to trade a specific contract. Keep in mind that your excess to trade changes along with your open trade equity.
Initial margin is the amount of money set aside to trade a specific contract.
The maintenance margin is the level at which a margin call is triggered. If the total equity in the account drops below the maintenance margin level you will be placed on a margin call and will need to deposit enough funds to bring you back to your initial margin level. Remember that margin is a reflection of risk. If the volatility in a market increases so can the margin. Margins are subject to change without notice and can be changed after you establish a position.