There are literally hundreds of varieties of lettuces in the market. Due to a wide range of growing regions from Salinas, CA to Yuma, Arizona (as well as several other western states) the majority of these lettuces are available year round. There are several different classifications of lettuces but they are generally divided them into two general groups: familiar head and large leaf lettuces (Iceberg, Butter, Romaine, Red Leaf, and Green leaf) and the smaller loose leaf lettuces (Tango, Lolla Rossa, Red and Green Oak Leaf, Red Perella and many more).
For many years, large leaf lettuces (Iceberg, Romaine, etc.) have been the major components of many standard table salads, used on their own and combined together with a multitude of different dressings. These varieties range in size, color (though usually some shade of green), and texture (from the compact, crunchy head of iceberg to the somewhat frilly leaves of red and green leaf or the elongated leaves of Romaine).
The smaller loose leaf type have leaves that branch from a single stalk and are more delicate and often have more flavor than the larger lettuces, though more perishable. They are also more colorful and offer more leaf shapes and textures.