Why was ReplicaSet introduced in Kubernetes?

ReplicaSet is usually not standalone, these are owned by Deployment. A single Deployment can have many ReplicaSets in its life cycle as new Version deployment added one more ReplicaSet.

Deployment allows us to rollback to previous stable releases if required.

Replicasets is updated version of Replication controller

In Replication controller there equality-based selectors

In Replicasets there is set-based selectors

Replicasets also work with deployment so when you do simple deployment in kubernetes replicasets automatically generated and managed.so deployment owned replicasets.

I think this can be summed up to the 3 points below.

1 ) ReplicaSet is a replacement for the Replica controller and supports richer expressions for the label selector. You can choose between four values of operators In, NotIn, Exists, DoesNotExist - see Set-based requirement.

2 ) There is a small differnece in the syntax - ReplicaSet which contains matchLabels field under the selector:

apiVersion: apps/v1kind: ReplicaSetmetadata:  name: nginxspec:  replicas: 3  selector:    matchLabels: #<-- This was added      tier: nginx

3 ) When you see Replica Controller is mentioned in one the docs or other tutorials - refer to it as ReplicaSet AND consider using Deployment instead.