The Japanese sweet potato is a low moisture, white flesh potato with red colored skin. Like the common yellow skin sweet potato it can be baked, mashed or fried. Even though it looks similar to what is called a "yam" in the U.S. it does not have the moist interior. common to those other varieties. Don't confuse this with the purple-flesh Okinawa sweet potato.
Look for these potatoes (frequently called a Japanese Yam) in many well-stocked grocery stores including Whole Foods markets. They make an excellent substitute for the yellow-skin potatoes when they are out of season.