Southwest China is the most exotic area of China, home to many minority ethnic groups. It has much in common with Northwest China; both areas are far less prosperous than the eastern and southern coastal regions.
Much of the terrain is quite mountainous. Tibet and Northwestern Yunnan are located on the Tibetan Plateau and have many very high mountains; including the world's highest (Mount Everest) which is located in on the Tibet/Nepalese border. Guizhou has smaller mountains, terraced rice fields, and many caves.
The regions of Sichuan and Chongqing are considered part of Western China, because they are the edge of "China proper" (the area historically inhabited by Han Chinese) even though they are near the geographical center of the modern Chinese state. Western Sichuan was once part of a Tibetan Empire and part of Eastern Tibet; it lies at the edge of the region and is the gateway to the frontier/minority provinces of Tibet and Qinghai.
Chongqing is historically and culturally part of Sichuan, but is now administratively a separate municipality and a mega-city in its own right. It is China's largest city, with a population of almost 30 million counting suburbs, countryside, and the many towns under its administration.