A teacup Chihuahua that is referred to as an "apple-head" has a well-rounded, apple-dome-shaped head and a short snout. Only Chihuahuas with this shape of head are recognized as a breed standard and are therefore qualified to be a show dog. The other head shape that some Chihuahuas possess is referred to as a "deer-head." This dog's head shape is more elongated, resembling a deer, with a longer snout.
There is also a type of Chihuahua referred to, usually just by breeders, as a "pear-head." This type of Chihuahua is considered undesirable simply because they do not look the part of an average Chihuahua. Their heads are a similar shape to deer-head Chihuahuas but with a flatter skull and a larger, more protruding snout. They also tend to be significantly larger than the average Chihuahua.
As far as the word "teacup" in reference to this breed, it is not actually a specific breed of Chihuahua. It's simply a term used by breeders to enhance the marketability of their puppies. Chihuahuas can range anywhere between two to eight pounds, and teacup is usually used to describe a Chihuahua on the smaller end of that scale. However, this size quality is not something that can be specifically bred for.