Lettuce Types

Crisphead--Also known as Batavian or Summercrisp. Makes a frilly, loose head with crisp midribs and hearts. Not the same as the round, tight-headed iceberg types (which require a cool climate). This is the most heat-resistant lettuce, and the only kind that whose seeds will sprout well in summer heat. Also resistant to cold. 
Romaine--Tall, dark green leafy heads with juicy midribs and crisp heart. Adaptable, nutrient-dense, and popular.
Butterhead--Soft leaves with a buttery texture and flavor make small round, roselike heads. Best in cool weather, though a few have been selected for summer.
Leaf Lettuce--The fastest to grow and high in nutrients, because the open shape means all the leaves get sunlight. Leaves can be wide or cut into oakleaf shape. Well-adapted to cutting a few leaves at a time rather than harvesting whole heads and replanting. More flavor than heading lettuce.