Guinea pigs have been specially bred to produce certain varieties.  The most common and accepted breeds are Abyssinian, American, Coronet, Peruvian, Silkie, Teddy,  Texel, and White Crested which are described in further detail below.  All these breeds except for Coronets and White Cresteds also come in satin, a shinier smoother coat.
Note: If you are a beginner to cavies and are trying to decide upon a breed,  I do not recommend Peruvians, Silkies, or Coronets because their long hair requires daily grooming.

Cavy Fanciers Poem describing the breeds

Color Varieties:

Agoutis
(Each hair shaft is 'ticked' two different colors)
Dilute
Golden
Silver

Marked
Broken Color
Dalmation
Dutch
Himalayan
Tortoise Shell
Tortoise Shell and White

Self
Beige
Black
Blue
Chocolate
Cream
Lilac
Red
White

Solid
Brindle
Dilute
Golden
Roan
Silver

Abyssinians are most recognizable by the ten rosettes of fur on their rump, hips, saddle, and shoulders.  The ridges these rosettes form make for a mohawk-ish look down the middle of an Abby's head.  They should also have a mustache of raised fur around their noses.  The fur all over their body is courser than that of most other breeds.

Photo coming soon

Americans have a smooth, straight coat and a full crown.  The nose is to be Roman (wide and curving) and the eyes large and spaced apart.  This breed is judged otherwise by its specific color variety.

Sorry for the poor picture quality.  Better breed standard pictures coming when I can raise enough money to fix my scanner :)

Coronets have longer hair and a single rosette on the top of their heads.  The hair from the back of the rosette grows back to join with that of the rear without a part down the center.  The head is short with a Roman nose.

Peruvians have thick long hair that falls about them evenly in the front and back.  The hair also is thick around the head.  Their shoulders are broud and their eyes large.

Silkies also have long hair but it is a littler finer and more shiny.
The hair from its head grows back to join with that of the rear without a part down the center. 

Teddies have a short, dense, wirey coat enough so that the fur should bounce back when patted.
The eyes are large and the skin firm.

Texels have long thick hair with little curls.  They are a little rougher than the other long hair breeds.

White Cresteds have a short smooth coat like that of the American and a single white rosette on the top of their heads.  They have a Roman nose and large bold eyes.  They are not accepted in Broken Colors, Dalmation, Dutch, Himalayan, Roan, Tortoise Shell and White, or White varieties.

Photo coming soon