Replica Set is the next generation of Replication Controller. Replication controller is kinda imperative, but replica sets try to be as declarative as possible.
1.The main difference between a Replica Set and a Replication Controller right now is the selector support.
+--------------------------------------------------+-----------------------------------------------------+
| Replica Set | Replication Controller |
+--------------------------------------------------+-----------------------------------------------------+
| Replica Set supports the new set-based selector. | Replication Controller only supports equality-based |
| This gives more flexibility. for eg: | selector. for eg: |
| environment in (production, qa) | environment = production |
| This selects all resources with key equal to | This selects all resources with key equal to |
| environment and value equal to production or qa | environment and value equal to production |
+--------------------------------------------------+-----------------------------------------------------+
2.The second thing is the updating the pods.
+-------------------------------------------------------+-----------------------------------------------+
| Replica Set | Replication Controller |
+-------------------------------------------------------+-----------------------------------------------+
| rollout command is used for updating the replica set. | rolling-update command is used for updating |
| Even though replica set can be used independently, | the replication controller. This replaces the |
| it is best used along with deployments which | specified replication controller with a new |
| makes them declarative. | replication controller by updating one pod |
| | at a time to use the new PodTemplate. |
+-------------------------------------------------------+-----------------------------------------------+
These are the two things that differentiates RS and RC. Deployments with RS is widely used as it is more declarative.