Margin
The deposit required to open or maintain a position. Margin can be either "free" or "used". Used margin is that amount which is being used to maintain an open position, whereas free margin is the amount available to open new positions. With a $1,000 margin balance in your account and a 1% margin requirement to open a position, you can buy or sell a position worth up to a notional $100,000. This allows a trader to leverage his account by up to 100 times or a leverage ratio of 100:1. If a trader's account falls below the minimum amount required to maintain an open position, he will receive a "margin call" requiring him to either add more money into his or her account or to close the open position. Most brokers will automatically close a trade when the margin balance falls below the amount required to keep it open. The amount required to maintain an open position is dependent on the broker and could be 50% of the original margin required to open the trade.
Leverage is the ability to gear your account into a position greater than your total account margin. For instance, if a trader has $1,000 of margin in his account and he opens a $100,000 position, he leverages his account by 100 times, or 100:1. If he opens a $200,000 position with $1,000 of margin in his account, his leverage is 200 times, or 200:1. Increasing your leverage magnifies both gains and losses.
To calculate the leverage used, divide the total value of your open positions by the total margin balance in your account. For example, if you have $10,000 of margin in your account and you open one standard lot of USD/JPY (100,000 units of the base currency) for $100,000, your leverage ratio is 10:1 ($100,000 / $10,000). If you open one standard lot of EUR/USD for $150,000 (100,000 x EURUSD 1.5000) your leverage ratio is 15:1 ($150,000 / $10,000).
Matterial is taken from http://www.goforex.net/forex-basics.htm
