Unless you've always lived in a city, at some point you've experienced a county fair, draft horses, cotton candy, beef cattle, giant pumpkins, and carnival rides.
Medieval people did not exactly have county fairs (and definitely did not have giant pumpkins), but they certainly had fairs. Their trade fairs had some of the same elements we now see in county fairs, but rather than showcasing agriculture, they were focused on long-distance trade.
The main fairs in the High Middle Ages were in the Champagne region of northeastern France. The drink champagne is named for the region, by the way, not the other way around; the drink required the invention of modern bottling and dates in the form we know it to the eighteenth and nineteenth centuries. (The name itself comes from the Latin Campania, just meaning countryside.)
The Champagne region is roughly halfway between Flanders, where raw wool from the British Isles was woven into cloth, and Italy, both the region where spices from the East entered Europe and the center of important cloth-dyeing. There were six fairs a year, six weeks each, that rotated among major cities of Champagne.
Here cloth (including silk, linen, and wool, both raw and dyed), leather, spices, furs, weapons, and all sorts of other products would be bought and sold, primarily wholesale. Huge warehouses were built where the merchants could deposit their wares. Banking developed at the fairs, as those arriving with money to buy goods needed someplace safe to put their money, and as those arriving with goods to sell might need a short-term loan to tide them over until they had sold enough. (Click here for more on medieval banking.)
Every fair had its own system of weights and measures, and cloth would have to be unwound from the bolts and measured, using the local yardstick. Spices were weighed using local scales; "troy weight" is still used for jewelry, and gets its name from Troyes, one of the chief Champagne cities, that had not just one but two fairs every year.
The local citizens profited from the fairs even if they were not merchants. Many had a room at the front of their house that they could close off from the rest, which they would rent out during the fair. Inns, prostitutes, entertainers, and farmers from the countryside selling food all enjoyed the influx.
The counts of Champagne also did very well off the fairs. They policed the fairs and all the roads leading to them, to protect the merchants from bandits, and to make sure that no big fights broke out and that no one was cheated. In return they collected sales tax and set up toll bridges.
Nonetheless, the fairs always were fairly rowdy. Sometimes as it gets dark, if one is walking down the midway of a modern county fair, with dark and light curiously mixed, and weird, certainly-fixed games inviting one to play, one can almost feel what seems like the beating of an evil heart. Medieval fairs felt it too.