The Silk Road is a metaphor for long-distance trade across Asia that first developed from around 300 B.C. to roughly 200 A.D. It was not, in fact, a "road," but rather a collection of land and sea routes linking cities, trading posts, caravan watering places, and hostels between the eastern Mediterranean and China. Afghanistan was centrally located along the major routes.
The distances to be covered were so great—and the rigors of travel so daunting—that only goods small in size and high in value could be transported the entire distance. Products were seldom carried from one end of the Silk Road to the other by the same merchants. Most products, however, were traded from hand to hand, from trading town to trading town. Goods were added to the caravans as they went through markets—silk and lacquer from China, ivory, rubies, and garnets from India, horses from Siberia and Mongolia, and carpets from Persia and the steppes, the vast grassland north of the Silk Road. Trade goods brought to China included precious metals, coins, glass, and semiprecious stones.
See the Heilbrunn Timeline of Art History to learn more about trade routes between the Romans and the empires of Asia.