Member of the cabbage family, collard and kale are virtually interchangeable in cooking. Kale books in about half the time, however, and consequently requires a little less water in the pot. Collard has and earthy flavour, while kale has a sharper, more pronounced cabbage taste. Collard is a wide, heavy, long stemmed, flat leafted brassica.
Image from: http://www.ahomesteader.com/