Collard greens are a member of the cabbage family (a cruciferous vegetable). The greens provide a fairly mild flavored green similar to kale. Collard greens are well known in the Southern states of the U.S. and they contain as much calcium as milk.
These greens are typically simmered for a long time, around 2 hours. A traditional Southern U.S. style would be to simmer the greens with some sliced bacon or a smoky ham hock and yellow onion and sometimes dried chile powder such as cayenne.
Alternately, Paula Deen offers this recipe for Quick Collard Greens which cooks in just about 6 minutes.
Collard greens are typically available all year round in well-stock grocery stores and some farmer's markets. In addition to fresh greens some stores stock frozen collards.