Yams so-called (USA only) were pushed into the market in the 1930s. Both the darker 'Louisiana yam' and the lighter 'sweet potato' grown in the USA are just varieties of each other.
The true yam needs a very long season and is imported for foreign food outlets, NOT grown here. It belongs to a different plant group entirely.
The name 'yam,' however, has pretty well pushed the name 'sweet potato' into the background.
See Sweet Potato
This Texas A&M University site also has some good recipes!