CRISPHEAD-(syn.'Iceberg', 'cabbage lettuce') this was virtually the only kind of lettuce in existance in the consciousness of the New Zealanders of the 50's and 60's . In the UK and USA, this type is referred to as 'iceberg' lettuce, as it is so crisp and has such a white, crisp, densely packed heart. Sometimes also known as 'cabbagehead' lettuce. Crisphead varieties bred specifically for summer growing are slow to bolt. Their main problem in summer is a slimy internal rotting.
 
Photo: a very good photo and some notes on lettuce types and how to store and use them are at this page.
http://www.trikaya.com/lettuce2.htm
 
SUMMER CRISP-(syn. 'French crisp', 'Batavian') unknown to many of us, yet one of the best lettuce types, Although variable between the various varieties, overall it is perhaps intermediate between the crisphead and the loose leaf lettuce. The outer leaves are thick, crisp and flavorsome, the often conic heart/head is crisp and sweet. In some varieties, the outer leaves can be harvested as a loose leaf until the head starts to form. These are quite large lettuces, and need good fertility. Their particular virtue, apart from their top rate taste, is their ability to 'stand' without bolting.

BUTTERHEAD-(syn.'boston', 'bibb') until fairly recently, the best known type of lettuce in the UK and very popular in USA. It is also the European lettuce. This type has thick but soft and tender leaves forming a cream coloured, soft, blanched heart. The outer leaves are large and ruffled. The very finest tasting lettuces, such as 'Nancy', are in this group. Butterheads are rarely bitter, quick maturing, more tolerant of less than ideal soil and weather conditions than crispheads, even if they don't 'stand' for a long time in the garden without bolting.

LOOSE-LEAF-(syn. 'leaf') Well known and appreciated in the USA, but for years relatively unknown in the Southern Hemisphere until the explosion in hydroponic lettuce growing. These are the easiest of all lettuce types to grow. They form no heart or head, and are, as the name implies, a cascade of loose leaves, which may pe picked without cutting the whole plant. There is great variety in this group-thick leaves, thin leaves, savoyed leaves like a silverbeet leaf, flat leaves, red, green, frilled, cut, curled, and so on. Some, such as 'royal oak leaf', are outstanding-thick, tender, delicious. Others, such as 'lollo rosso', look great on a plate, but are uninspiring eating. As a group, they tend to bolt more quickly in heat than other types.

COS-(syn 'romaine') Supposedly originally from the island of Cos in the Mediterranean, this type of lettuce is very upright and columnar, forming a particulalry sweet creamy white crisp heart, surronded by sturdy outer leaves. Fairly demanding to grow, in terms of being unforgiving of poor conditions (a bitter reward), and poorly adapted to hot weather. Some claim this type to be the sweetest and best tasting of all when grown well. All things being equal, in hot weather, the crispheads are the slowest to bolt, then the summercrisps, the bostons, the leaf lettuces and finally the cos types.
 
Photo: a very good photo at a major vegetable exporters site.
http://www.trikaya.com/romaine.htm
 
An asterisk (*) indicates the variety can also be grown in summer.

CRISP HEAD
*GREAT LAKES-dependable solid, bolt resistant
*WEBBS WONDERFUL-large hearted, frilly leafed, dark green, adaptable, bolt resistant.
*SALADIN (syn 'Salinas)-uniform, vigorous, large firm mid green head, rot resistant, bolt resistant.

SUMMER CRISP
*CENTENNIAL- performs well summer and winter, centennial looks very similar to a criphead type, but the outer leaves remain crisp and edible, while the head is greener and with more flavor than a crsiphead. It 'stands' in the garden very well, even in hot weather.
VICTORIA-a very vigorous, large, and upright lettuce, the outer leaves are deep green, crisp thick and juicy, the loose heart is semi blanched, crisp and sweet. Victoria resists tip burn and bolting.

BUTTERHEAD
*BUTTERCRUNCH-a small lettuce, fast maturing, with very green outer leaves enclosing a small, compact, fully blanched, sweet, crisp and tender heart-almost crisphead-like. It has very good flavor.Bolt resistant.
*DEERS TONGUE tonque shaped, light green leaves with goodflavour and texture- Bolt resistant.
*DOLLY-a quick growing, medium sized, light to mid green, butterhead with tender, smooth leaves, and very good bolt resistance.
MERVEILLE des QUATRE SAISONS -syn 'Little Red Riding Hood', 'Besson Rouge", 'almost magenta coloured outer leaves, contrast with the pale green to cream colored heart . A well known 'red' butterhead, of which 'carmona' and 'sangria' are basically improved versions-heavier, more bolt resistant, better color.
PERELLA-RED a small (15-18cm wide ), rather open Italian 'heirloom', green blushed red where the leaves are exposed to the sun.
SANGRIA  Thick rose blushed leaves over a blanched pale yellow heart make for an attractive lettuce and very good eating quality. Sangria is medium sized bolt resistant and tip burn resistant.
TOM THUMB-very small, pale to mid green, tightly hearted butterhead about the size of a tennis ball. Fast maturing and very good flavor. While it tends to run to seed in summer, it can be grown as a fast growing summer lettuce (even in pots) so long as it has excellent fertility and moisture

LOOSE LEAF
BLACK SEEDED SIMPSON -a quick producing loose-leaf with slightly savoyed and frilled crisp and juicy light green leaves.
GRAND RAPIDS T.B.R Compact, light green, with heavily frilled and curled leaves. Grand Rapids resists tip burn (scorching of the leaf margins).
*LOLLO ROSSO a highly attractive lettuce, compact lettuce with heavily frilled pinky bronze leaf margins. It looks great as a garnish, but it is very mediocre eating compared with what else you could grow instead.
'LOVINA' is a recent, more bolt resistant version.
*LOLLO BIONDO-A bright green version of lollo rosso.
*LOMA A particularly small frilly edged, deeply toothed loose-leaf, tip burn and bolt resistant.
*OAK LEAF like salad bowl, oak leaf has particularly thin and soft leaves. Noted for it's resistance to bitterness. The plant forms an attractive, light green mound of unusual shaped leaves. It is said to need only 4 hours sunshine a day, which would make it very versatile in a small space garden. Bolt resistant.Very good eating quality.
PRIZE HEAD This is a leaf lettuce, not a heading lettuce. It has bronzy red outer leaves over frilled lime green inner leaves
*ROYAL OAK LEAF- a darker green and more heat tolerant version of Oak Leaf; standing longer without the bottom leaves yellowing.
RED FIRE a large, red, ruffled loose-leaf similar to-
RED SAILS, which it replaces. Bolt resistant.
*RED SALAD BOWL  A bronze-red version of salad bowl.
ROSSIMO -An open plant with blistered and savoyed leaves and fringed leaf margins. Bright red.
*SALAD BOWL -syn 'green salad bowl'-lime green, thin, tender, deeply and delicately lobed leaves form a vigorous rosette that resists bolting fairly well.Exceptionally good eating quality, but very soft, so has to be used relatively quickly or it wilts.
SIMPSON ELITE-A bolt resistant, reselected form of 'Black Seeded Simpson'.
'TANGO  A stunningly attractive bright green deeply cut leaved lettuce. The plants are dense, compact and uniform. It looks fantastic on a plate, but the eating quality is unremarkable

COS
DIAMOND GEM a selected form of ' Little Gem' .
LITTLE GEM syn 'Sucrine'a very small cos that has some elements of the butterhead in it. The approx 15cm tall plants have tightly folded outer leaves over a compact and dense interior. It  is sweet  and  quick maturing.
ROSALITA Rosalita is upright, medium size for a cos, and is deep purple over a yellow blanched interior. It must be grown well in good conditions to avoid biterness.
ROUGE D'HIVER An heirloom European variety with bronzy red outer leaves over a solid heart. Matures after 'Rosalita'.
PARIS WHITE  The standard main season Cos. The large plants are about 20-25cm tall, cylindrical and upright.The light green slightly savoyed outer leaves are tightly self folded over the white blanched heart. Matures after 'Rouge d'Hiver'.