What's the difference between sweet potatoes and yams?

True yams are root vegetables grown in central Africa, they are round and large and very dry in texture, and are the reason that so many humans can be fed in such an overpopulated area. They have a chemical in them that is said to be responsible for Nigeria having the highest per capita amount of twins. Sweet potatoes are also root vegetables but are sweeter, have more moisture and come in a variety of colors. There are orange, yellow, white, and purple-fleshed varieties of sweet potatoes. The darker colors have a higher rate of antioxidants and the orange or yellow ones have high rates of beta carotene.

I believe their the same family...yet yams are sweeter