If you are referring to the Dataiku Cloud
1) Plugins would be the existing plugin available in the Plugin store :
https://www.dataiku.com/product/plugins/ Note some may not be visible in the launchpad or compatible/applicable to Dataiku Cloud.
2) Extensions, on the other hand, would be used to install specific features like Automation or API nodes, Git integration or Spark-on-K8s etc. These actions would be performed from Administration for Installed DSS but as there is access to no Administration these tasks are performed from Extensions.