Traders can regularly make a fortune for their employers
Trading is one of the most coveted jobs in the financial markets.
A good trader can make tens of millions of dollars for his company every year and take home vast bonuses.
It is a stressful job. One wrong move and your profits can be wiped out, your reputation destroyed and your job gone.
Traders at the big investment banks such as Societe Generale will be organised into units or desks, based around different financial products including bonds, stocks and currencies.
Using the bank's funds, the more experienced traders will be able to take positions - or bets - on which way they think the price of a particular product is going.
A common way of doing that is a futures contract.
Simply put, a trader will promise to buy or sell a product at a certain price at a certain time.
For example, if the trader thinks the price of oil is rising, from today's price of $75 a barrel, he might promise to buy oil at $75 in 3 months' time.
If he is right and oil hits $100, then he has made $25.
But there's a problem.
If oil falls to $50, then he will have to pay the difference, $25. Or worse, it could fall to $25 and he's lost $50.
In reality, a fast-changing market means a trader can see his profits wiped out in seconds and his position quickly plunge into the loss-making zone.
So banks have checks to make sure they do not get into serious trouble. Traders will have managers who will monitor their staff's activities.
If they think a position is too risky, they will tell the trader to close it.
There is also a unit known as the "back office". These are non-trading staff who calculate the day's trading and try to work out how much the bank is making or losing.
Senior managers will look at those numbers and make sure the bank is not taking on too much risk.
But the system can break down, as in the case of Barings in 1995.
There, Nick Leeson was able to hide huge losses, as he was the trader and manager and looked after the back office.