Beet greens: Taste much like beets, are fairly tender and cook quickly by steaming, stir-frying or sauteing.
Collard greens: One of the tougher and more strongly flavored greens. Before cooking, blanch quickly in simmering water to tone down bitterness. Traditional in Southern cooking.
Kale: A member of the cruciferous vegetable family. Related to cabbage, kale may be the oldest known green. Comes in many varieties that are usually not bitter, especially when winter weather sweetens them.
Mizuma: A Japanese green with long, pointy leaves, mizuma is one of the mildest greens. Can be used in salads or added to other greens when sauteed or stir-fried.
Mustard greens: Comes in red or green. A tough, spicy cruciferous green often used in Southern cooking. A terrific flavor booster when young leaves are added to a salad.
Young red mustard leaves add a nice color to salads.
Spinach: Good raw or cooked and may be substituted for many other greens in recipes, especially where color is important. Cooks more quickly than tougher greens such as beet, kale, chard and collards.
Swiss chard: Comes in red or green. Tastes like beet greens; red variety is usually more tender. Separate the leaves and stem and cook the tougher stems first. Large leaves are great for stuffing. Can be used interchangeably with spinach or beet greens in most recipes.
Tat soi: An Asian green with small rounded leaves that grow in a rosette pattern. Not bitter, they make great decorations, can be sauteed whole or with leaves separated, and can be used in salads or added to sautes and stir-frys.
Turnip greens: One of the heartier greens, somewhat spicy with a turnip taste.
Remove leaves from stems and discard stems. Use interchangeably with kale, Swiss chard or beet greens, depending on other flavors in the recipe. If really tough, these greens may benefit from blanching before cooking.
Source: "The New Whole Foods Encyclopedia: A Comprehensive Resource for Healthy Eating " by Rebecca Wood