Explain the concept of CRD (Custom Resource Definition) in Kubernetes and how it is used to extend cluster functionality.

CRD (Custom Resource Definition) in Kubernetes allows you to create your own resource types, extending the standard set of Kubernetes objects (e.g., Pod, Service). This enables describing and managing application-specific or organization-specific resources via the Kubernetes API.

When you create a CRD, you define a new entity with its own set of fields. After that, Kubernetes starts recognizing these objects as full-fledged resources, which can be managed through kubectl, manifests, and API.

Example of usage: if you have a specific object for your application, such as "BackupJob," you can create a CRD for it and manage such jobs like regular Kubernetes resources.

This extends the cluster's functionality, allowing integration of custom logic and automation, often in conjunction with controllers (Custom Controllers) that monitor the state of these resources and perform necessary actions.