Teacup Cats

Teacup cats are simply small miniature cats (meaning a size smaller than the category of cat called "miniature cats"). Miniature cats are usually about one third to one half the size of normal size cats of the same breed. As a guideline teacup Persian females weigh 2-4 lbs while the males are 3-6 lbs.
At least one cattery sub-classifies by referring to "Palm Sized Pocket Persians" as well. Female palm sized weigh 3.5lbs or less and males 4lbs or less.

They are normally less than 9 inches tall or less when mature. Miniature kittens are more vulnerable than normal sized kittens. Being miniature poses health problems to pregnant cats so the breeding females must be 4-8 lbs in weight and breeding males are 4-10 lbs. It seems that they are friendly (to humans and dogs!) cats.

There is a nice story on the internet about a person called John Antrobus, who used to breed teacup cats (he may still do it but a search proved fruitless).

He first discovered miniature cats in Argentina. He says that he found them surviving in a refuse dump, in a back alley. He decided to bring 7 home to Canada and 6 survived the trip (one died due to the sedative needed for the journey).

He bred them successfully. He says that a "trade off" for the small size is that they are short lived (although this probably only applied to the breed he was dealing with).

Due their small size it seems that it is impractical to home them before 5 months old. Otherwise they are normal healthy kittens.
Sarah Hartwell, an expert on cats and particularly cat genetics, says that you should take care when acquiring very small cats.