What is Kubernetes StateFulSet?
As name says statefulsets : In kubernetes if you are running the stateful application that time instead of deployment you have to use the statefulsets.
Stateful sets are used for application storing data in memory, session and handling state.
For example stateful set can be useful for Elasticsearch, Redis
Yes, it is similar to a replicaset but with some extra features like ensuring that only one pod is created at time and allowing a templated volume config so each pod gets different volumes (unlike a RS where the pod template results in every pod being identical). These features are aimed specifically at stateful applications like databases, though the specifics would depend on the database and are out of scope for an SO question.