Barter

Bartering directly trades goods and services without relying on a medium of exchange. While it's more direct, it requires a "double coincidence of wants," which is to say that you want something of theirs and they want something of yours that is equal in value to them. This drastically reduces the people you can do business with, since everyone doesn't have exactly what you want.

Without a widely accepted currency, old civilizations often relied on barter systems to trade goods and services. The creation of currency and foreign exchange markets has removed the need for the double coincidence, and has facilitated trade between countries in the modern day.

If I have a cow and you have medicine, I might want to get rid of my headache and you might want some meat on your table. However, if my headache isn't too bad, I might value my cow with all the meat and milk it produces over an Advil. On the other hand, if I desperately need the medicine to save my life, you might ask for more than just one cow because of how valuable the medicine is.