Overview
The purpose of a ReplicaSet is to maintain a stable set of replica Pods running at any given time. As such, it is often used to guarantee the availability of a specified number of identical Pods.
How it Works
A ReplicaSet uses a selector to identify the Pods it should manage. If there are too few Pods, it starts more. If there are too many, it deletes the extra ones.
Usage
While ReplicaSets can be used independently, they are almost always managed by a Deployment, which provides higher-level features like rolling updates and rollbacks.