Actually,shortselling is the most interesting form of 'business' you can do.
what has been illustrated by Jammo and Mainat about how it works is correct. Just to add a point on Mainat's question on what happens to the real owners:
What really happens in the markets where this is allowed is that customers are allowed to buy shares on credit. hence you can go to AIB,buy 10,000 shares of equity but only pay for 5,000 shares. AIB then funds the other 5,000 shares but it remains in their name for security purposes but it is ideally yours. The AIB funded 5,000 shares is what the trader goes to borrow. this is because,you as the 'owner' of 10,000 equity shares,has only paid for 5,000 shares. so even if it got lost,you wont necessarily loose that much. but because you are paying an interest on this borrowed 5,000,the broker has to ensure that he keeps the shares for you. Mostly,these brokers know what kind of an investor you are. If you are like Jammo,they wont touch your equity shares because they know you will be coming for them any minute,if you are the long haul guys,they play the game with your 'borrowed' shares. Should you requier your shares and the broker has already lent them out,then he goes to another broker and borrows or allocates other shares from another customer but these shares have to be in the 'borrowed' category i.e,the customer had also not paid for them,in similar terms. sometimes they may make a loss,but mostly they do make profits and they are cautious the time frame they give. Mostly,its only the broker who can lend coz he is the one who holds the 'borrowed' shares.
I hope this makes alittle bit of sense.
Remember to read this together with Jammo's and Mainat's explanations.
Some deals are like glass. Sometimes it's better to leave them broken than try to hurt yourself putting it back together.