Merge pull request #43136 from pegasas/api-extension

Add documentation explaining the difference between CRDs and API aggr…

content/en/docs/concepts/extend-kubernetes/api-extension/_index.md

@@ -2,3 +2,18 @@
 title: Extending the Kubernetes API
 weight: 30
 ---
+
+Custom resources are extensions of the Kubernetes API. Kubernetes provides two ways to add custom resources to your cluster:
+
+- The [CustomResourceDefinition](/docs/concepts/extend-kubernetes/api-extension/custom-resources/)
+  (CRD) mechanism allows you to declaratively define a new custom API with an API group, kind, and
+  schema that you specify.
+  The Kubernetes control plane serves and handles the storage of your custom resource. CRDs allow you to
+  create new types of resources for your cluster without writing and running a custom API server. 
+- The [aggregation layer](/docs/concepts/extend-kubernetes/api-extension/apiserver-aggregation/)
+  sits behind the primary API server, which acts as a proxy.
+  This arrangement is called API Aggregation (AA), which allows you to provide
+  specialized implementations for your custom resources by writing and
+  deploying your own API server.
+  The main API server delegates requests to your API server for the custom APIs that you specify,
+  making them available to all of its clients.