Butter Lettuce - [Butterhead, Boston, Bib or Limestone]
These are very tender loose leaf lettuces often packed in individual plastic containers to protect them from damage. Of the two main varieties, Bib Lettuce is the smaller and considered the more flavorful, but Boston Lettuce is a suitable substitute. Packages I've seen in Southern California do not use either designation but simply call the contents "Butter Lettuce". The package often includes roots and a hydroponic growing matrix to extend shelf life.

Green Leaf Lettuce
Green Leaf is too tender to be a good industrial product but it's a very fine base for quality salads. Plenty of lettuce flavor with very moderate bitterness and good color. The photo specimen, squished a bit flat in shipping, as they always are, was 14 inches wide, 10 inches from stem to tip and weighed 13 ounces.

Iceberg Lettuce
This head lettuce is the darling of the lettuce industry and the fast food chains. It's easy to handle and stands up well to processing and shipping abuse. The fast food people like it because it's easy to shred, bulks up a salad well and stays crisp and bulky for quite a while after shredding. It has rather little lettuce flavor or bitterness but can be useful for recipes that call for lettuce wedges and such or where texture is more important than flavor. It's also easy to store in the fridge compared to leaf lettuce.. The photo specimen, a somewhat smallish one, was 5 inches diameter and weighed 1-1/3 pound.

Red Leaf Lettuce
Very similar to Green Leaf Lettuce except for the color. For taste and texture the two are interchangeable. The color is caused by anthocyanin pigments which are the same antioxidants found in red cabbage and red wine.

Romaine Lettuce - [Cos Lettuce]
The second most common lettuce in America after Iceberg and the sturdiest of the leaf lettuces. It's shape and stiffness make it relatively easy to handle but it does have quite a bit more lettuce flavor than iceberg does. It is called for by many recipes where the leaf shape and stiffness is desirable. The photo specimen was 13 inches high. It is often used as a foundation layer on a platter with the featured items placed over it.

Ruby Lettuce
Pretty much the same as Red Leaf Lettuce except for being all deep red with almost no green. For taste and texture the two are interchangeable. The color is caused by anthocyanin pigments which are the same antioxidants found in red cabbage and red wine.

Wild Lettuce
This is where it all came from, a wild plant with a bitter milky sap. When the plant matures it will "bolt", sending up a central stalk topped with a group of small dandelion-like flowers. The bitter sap is a mild opiate, particularly when the lettuce bolts.