Kale is considered to be a highly nutritious vegetable with powerful antioxidant properties; kale is considered to be anti-inflammatory. Kale is very high in beta carotene, vitamin K & C, lutein, zeaxanthin, and reasonably rich in calcium. As with brocoli and other brassicas, contains sulforaphane (particularly when chopped or minced), a chemical believed to have potent anti-cancer properties.
Until the end of the Middle Ages, kale was one of the most common green vegetables in all of Europe. Curly leafed varieties of cabbage already existed along with flat leafed varieties in Greece in the fourth century BC. These forms, which were referred to by the Romans as Sabellian kale, are considered to be the ancestors of modern kales. Today one may differentiate between varieties according to the low, intermediate, or high length of the stem, with varying leaf types. The leaf colours range from light green through green, dark green and violet-green to violet-brown. Russian kale was introduced into Canada (and then into the U.S.) by Russian traders in the 19th century.
During World War 2, the cultivation of kale in the U.K. was encouraged by the "Dig for Victory" campaign. The vegetable was easy to grow and provided important nutrients to supplement those missing from a normal diet because of rationing.
Kai-lan, a separate cultivar of Brassica oleracea much used in Chinese cuisine, is somewhat similar to kale in appearance and is occasionally called "kale" in English.
Info from: http://en.wikipedia.org/wiki/Kale