CryptoKitties is a game built on the Blockchain. The supply of kitties is limited by certain factors such as reproduction rate, which decreases every time they are bred. The first generation requires under an hour of rest time after having a kitty; After 6 breeding cycles, the slowest generation takes 1 week between each breeding cycle before they can reproduce again.
The kitties can be bred with one another to create one CryptoKitty. The baby kitties are born into with a combination of their parents' genetics.
The CryptoKitties have complex personalities and profiles describing their occupation, hobbies and interests.
CryptoKitties had >11% of the total ETH volume in the days following the release. It was a viral game that spread in popularity rapidly.  this was worth close to $100 million in volume within 24 hours. 
Because the genome is 256 bit, there are roughly 4 billion combinations of crypto kitties. The genetic code is a long hash code and it controls things like tail size, hair style, facial gestures, stripes, spots, whiskers and even the background of the kitty.