Answer:
Leverage let you buy more shares with less money. For example if you want to buy 100 Barclay's shares at 200p you would need £200 to do so, where's if you have leverage (let's say you pay 10%) then you would need only £20 to buy them.
Where's leverage increases your earnings potential it also increases your losses.
Try http://www.IndependentInvestor.co.UK for more information on leverage and available brokers.