The Spread represents the difference between the ask and the buy price. It is the profit for the market maker at the stock exchange that took on the risk of filling an order with his/her company’s money. Ideally the smaller the spread the better for you
Roll over is a term used when the action of closing out a position in an asset for one delivery date and reopening the same position for another delivery date. Retail forex traders often have an automatic rollover performed by their online forex brokers for currency pair positions left outstanding in their accounts by 5pm EST to avoid delivery.
Leverage refers to a process whereby the forex trader can borrow money from the dealer or broker at a certain ratio in order to be able to control larger positions in a currency pair. The concept of leverage provide a means for a group of disadvantaged traders to get access to cheap funding to enable them to hold large positions in the market.
The difference between the price at which an investor expects a market to be filled and the actual price of the execution is slippage.