Collards, as MMario pointed out, are of the brassica (cabbage) family. Of all cultivated brassicas, collards are the most "primitive", meaning that they most closely resemble the wild ancestor of our modern cabbage family crops. They are particularly well-adapted to the southern US climate, being more heat-tolerant and disease-resistant than most brassicas. They are usually planted in the fall and grown over the winter for an early spring main crop. Home gardeners usually pick individual leaves from several plants instead of harvesting entire plants, which provides a longer harvest.

Here's how I cook 'em: Chop up a few slices of bacon and cook them in a large pot to release the magic ingredient, bacon grease. While the bacon is cooking, remove the tough central leaf vein from the collards. When the bacon grease is ready, wash the collard leaves and toss 'em, still wet, into the pot. That should provide all the water you'll need to steam 'em well. As you cook them, they'll turn a bright green (from the heat causing cell walls to rupture which releases chlorophyl) and then fade to a more subdued color. I turn the heat off at that point. They're still a bit crunchy, but that's how I like 'em. Cook 'em longer if you like collard mush.