The sweet potato, a yellow or orange tuber, is a root of a vine in the morning glory family. The yam, native to Africa and Asia, is the tuber of a tropical vine related to grasses and lilies. They are two different botanical families and are not even distantly related.
Sweet potatoes are native American plants that were the main source of nourishment for settlers, homesteaders and soldiers during the Revolutionary War. They were even eaten at the first Thanksgiving. Today they are the sixth principal world food crop.
When Columbus landed on America’s shores in 1492 he found Native Americans growing sweet potatoes. He found them so tasty he took them back to Europe. In 1543 Spanish explorers found sweet potatoes growing in "Indian gardens" in what became Louisiana. They were being cultivated in the Carolina area long before European colonization there. The word for them was batatas, which eventually became patata in Spanish, patae in French and potato in English. (Around this time the word potato referred to the sweet potato, not the generic white potato as it does nowadays. It was about 1740 that the modifier sweet was added to that potato to differentiate it from the white potato that came here with Irish immigrants.)