In general, lettuces are greens used mainly in salad. There are hundreds of different varieties of lettuce, most of which are green and leafy, though colors can range from purple-red to a pale green that's almost white. Textures, too, can range from crisp and refreshing to buttery and soft. Most lettuces have a mild flavor, though some may be more peppery than others. Lettuce is generally classified by how its head is shaped: loose-leaf lettuces (also called cutting lettuces) don't have much of a head to speak of. Instead, they're a collection of soft, thin leaves that range from notched and scalloped to frilly and fluffy; oak leaf lettuces are a good example. Crisphead lettuce, has a tight head and crisp and thick leaves, iceberg being the prime example, though there are other varieties. Butterhead lettuces, such as Boston, has leaves that form a much looser head. Romaine has an elongated head with leaves that are crisp, sturdy, and sweet.
Escarole, spinach, and arugula are among the greens that can substitute for lettuce, escarole for the more firm varieties and spinach for the looser leaf varieties. However, flavors and textures will differ some. A crisp vegetable, such as sweet onion, Napa cabbage, or cucumber can give a sandwich a crunch similar to iceberg. Sometimes, however, there is no substitute: Romaine is the only lettuce you should use in a Caesar salad.
Look for heads or leafy rosettes that are dense and heavy for their size. Leaves should be unblemished ideally.
Remove and discard any outer wilted or discolored leaves and wash and dry well.
Keep lettuce in the crisper drawer of the refrigerator and try to use within a few days of purchase.