There are hundreds of varieties of lettuce grown throughout the world and, because they peak at different times of year, there's always a plenitude of this universal salad favorite. Of the lettuce used in this country, there are four general classifications-butterhead lettuce, CRISPHEAD, leaf lettuce and ROMAINE, most of which comprise many varieties. When shopping for any kind of lettuce a general rule of thumb is to choose those that are crisp and free of blemishes. As with all greens, lettuce should be washed and either drained completely or blotted with a paper towel to remove any excess moisture. A SALAD SPINNER, which uses centrifugal force to remove water from leafy greens, is a real timesaver for this process. Never allow lettuce to soak, as the water tends to soften some leaves. Refrigerate washed-and-dried greens airtight in a plastic bag for 3 to 5 days, depending on the variety. All lettuce is low calorie and most of it is rich in calcium, iron and vitamins A and C. Keep in mind that the darker green leaves contain the most nutrients. For information on specific varieties of lettuce, see individual listings.