Straight from Wiki -

"A contract for difference (or CFD) is a contract between two parties, typically described as "buyer" and "seller", stipulating that the seller will pay to the buyer the difference between the current value of an asset and its value at contract time. (If the difference is negative, then the buyer pays instead to the seller.) In effect CFDs are financial derivatives that allow investors to take advantage of prices moving up (long positions) or prices moving down (short positions) on underlying financial instruments and are often used to speculate on those markets."


The thing about CFDs(and spread betting) is that you can hold leveraged positions open on margin. So rather than buying £50k worth of stock, you'd be able to trade a £50k contract on, say, a 10% margin, meaning the margin requirement on the contract would be £5k. So in theory, all the capital you'd need in your account to trade the £50k contract would be £5k and something to cover any unrealised losses(which could be over £5k itself depending on how you did things). That would leave the account totally over-exposed, but you get the idea of what's possible. 10% margins and under are only for the most liquid stocks really. As you get outside the FTSE you'll see that change dramatically.

In the example above you could, in theory, wipe out the £5k required to keep the position open if the stock fell 10%. I'm guessing when you made £75k on Desire you were using way too much leverage for your account and, if things had gone against you, you could just as easily have lost £75k, or 3 quarters of your account capital to put it another way!