There are hundreds of varieties of Cucumbers, but they can be generally divided into two categories: slicing and pickling. "Slicing cucumbers" are meant to be eaten fresh and are larger with thicker skins. "Pickling cucumbers" are produced for eventual processing into pickles and are usually smaller with thinner, rough, and knobby skins.

You'll find all these varieties and more at the farmers' market, just-picked and full of flavor and nutrition.
Armenian cucumbers are long and thin, about 10-15 inches long.
Lemon cucumbers are pale yellow, round, and mild in flavor.
English cucumbers are long, slim, and seedless.
Persian cucumbers are small, slightly sweet, seedless, and burpless.
Garden cucumbers are the big seeded varieties, very common.
Kirby cucumbers are typically called pickling cucumbers, short and bumpy.