Yams originated in Africa and Asia. They are tubers and are related to lilies.
Sweet potatoes came from Central America. They are storage roots and are from the morning glory family.
Yams have bark-like skin which is brown to black. Their flesh can be an ivory colour, yellow or even purple.
Sweet potatoes are elongated with tapered ends. They have a smooth, thin, edible skin that can be white, yellow, red, purple or even brown. The flesh colours can be white to yellow, orange, or orange-red.
Yams have a starchy texture which is dryer than the flesh of a sweet potato, and they have less flavour.
Sweet potatoes can be categorized as firm or soft. Firm are white to yellow varieties which remain firm when cooked. Varieties that are considered to be soft are moist and soft when cooked. They fluff up when mashed. These are the red, purple and brown varieties.
Yams have a starchy taste, but they are easily jazzed up with hot sauces, herbs or spices.
Sweet potatoes are sweeter than yams. The orange fleshed sweet potatoes are sweeter that the firm white fleshed sweet potatoes.
That Yam You're Eating Is A Probably Sweet Potato Video