The correct answer is - money.
The bartering is a traditional and very old method in the economics. It is a method based on the exchange of one good for another good, proportionally to their value of course. This method has been largely abandoned throughout the developing and well developed countries, but it can still be found in use in the less developed countries.
In the today's economies though, the much more used method is the exchange of money for goods and services. The money have been invented in order to be an easy and practical method for trade since few thousand years ago, and their power and importance had steadily increased as the time was passing.
