Who
The Terracotta Army was made for and under the order of China's first Emperor Qin Shi Huang-Di. There was around 700,000 workers who all had made some contribution when making the warriors/army. But after the entire project had been finished, all of the workers had been killed to keep the army/mausoleum a secret and for it to be kept underground/unharmed for as long as possible.
What
The Terracotta Army was a platoon of clay sculptures that were buried alongside China's first emperor, Qin Shi Huang-Di. Many thousands of warriors were found, each with different/unique facial expressions, clothing, hairstyles and gestures. Terracotta is a kiln baked clay used to make an unglazed ceramic clay.