One thing you may be thinking you will be able to do with a ReplicaSet is rolling upgrades and rollbacks. Unfortunately, ReplicaSets can only replicate the same version of a pod; luckily, this is where deployments come in.
A deployment controller is designed to update a ReplicaSet or pod. Lets use NGINX as an example. As you can see from the following definition, we have 3 replicas all running NGINX version 1.9.14:
apiVersion: apps/v1beta1kind: Deploymentmetadata: name: nginx-deploymentspec: replicas: 3 template: metadata: labels: app: nginx spec: containers: - name: nginx image: nginx:1.9.14 ports: - containerPort: 80