Although in English, people often call sweet potatoes yams, in French they are two separate things (patate douce and igname). The confusion dates from the 18th and 19th century, when African slaves called the sweet potatoes native to the Americas ‘nyami’ which became ‘yam’.
Yams (from the Dioscorea family—there are 870 varieties) come from the humid tropics, and can be much larger than sweet potatoes. The igname sold in France (Dioscorea polystachya) or Chinese yam is long and white inside. They introduced in the 19th century, when the French potato crop was threatened with blight.
Image by Jurema Oliveira on Wikimedia Commons