Yams are a very important vegetable in the Igbo culture. They are the main source of nutrition and have even been dedicated deities such as Njoku Ji, the "yam god." They are also celebrated with an annual yam festival. Their yam is to our potato. The word yam originates from the African words njam, nyami, or djambi, meaning "to eat."
Although Yams are commonly mistaken for being the same vegetable as what we know to be the sweet potato, the two are not interchangeable. The yam is a tuber of a tropical vine, the Dioscorea batatas. Yams also contain a higher moisture and natural sugar content, tasting sweeter than the sweet potato.
While the sweet potato is a popular vegetable in the American South, the yam is more popular in Africa, Latin America, and the Caribbean.
For more information regarding the differences between yams and sweet potatoes, click this link.