A bobblehead cat is one that was born with a neurological disorder called Feline Cerebellar Hypoplasia. It is caused by the mother cat's infection with feline distemper virus or an injury to the kitten while small.
The severity of this disorder can be mild, like Betty, to severe. The problem doesn't get worse as the cat grows older. Cats who have a severe case of this disorder are often unable to find homes and are put down.
The vet who diagnosed Betty was surprised in later visits to see that Betty still had all her teeth. Often, these cats fall on hard surfaces and break teeth. They are unable to jump because their coordination isn't good enough to get them where they would need to go.
With Betty, she has always appeared to mentally be very sharp and is very opinionated. Betty Tales contains stories which show her disability and how she copes with it.
For more information on cats with Cerebellar Hypoplasia and links to adoption information and some wonderful stories, I love the website at Life With CH Cats (www.lifewithchcats.com).
There's also a great answer site at www.chcat.org. There, they'll answer all the common questions about CH cats.
For adopting one of these special cats, there's a terrific list at lifewithchcats.com/adoptable-ch-cats/