How to: What is a PetSet or StatefulSet?
In a regular Deployment all the instances of a pod are exactly the same, they are indistinguishable and are thus sometimes referred to as "cattle", these are typically stateless applications that can be easily scaled up and down. In a PetSet, each pod is unique and has an identity that needs to be maintained. This is commonly used for more stateful applications like databases. 

Learn more: http://kubernetes.io/docs/user-guide/petset/

In 1.5, PetSets have been renamed to Stateful Sets

Learn more: http://kubernetes.io/docs/tutorials/stateful-application/basic-stateful-set/