Chihuahuas are very small dogs, and are the smallest breed recognized by some kennel clubs. There are two main varieties recognized by kennel clubs, the short-haired and the long-haired. There is a second varietal split as well, having to do with the shape and size of the dog's head. These two descriptive classifications are "apple head" and "deer head", but only the apple head is conformationally correct.

source:
pic:
wikipedia: