Explain env vs envFrom for page Define Environment Variables for a Container (#40055)

* 'Define Environment Variables for a Container' does not explain env vs envFrom #39873

* suggested review changes done.

* tengqm suggested changes done.

Author: KhushPatibandha

content/en/docs/tasks/inject-data-application/define-environment-variable-container.md

@@ -21,6 +21,22 @@ When you create a Pod, you can set environment variables for the containers
 that run in the Pod. To set environment variables, include the `env` or
 `envFrom` field in the configuration file.
 
+The `env` and `envFrom` fields have different effects.
+
+`env`
+: allows you to set environment variables for a container, specifying a value directly for each variable that you name.
+
+`envFrom`
+: allows you to set environment variables for a container by referencing either a ConfigMap or a Secret.
+ When you use `envFrom`, all the key-value pairs in the referenced ConfigMap or Secret
+ are set as environment variables for the container.
+ You can also specify a common prefix string.
+
+You can read more about [ConfigMap](/docs/tasks/configure-pod-container/configure-pod-configmap/#configure-all-key-value-pairs-in-a-configmap-as-container-environment-variables)
+and [Secret](/docs/tasks/inject-data-application/distribute-credentials-secure/#configure-all-key-value-pairs-in-a-secret-as-container-environment-variables).
+
+This page explains how to use `env`.
+
 In this exercise, you create a Pod that runs one container. The configuration
 file for the Pod defines an environment variable with name `DEMO_GREETING` and
 value `"Hello from the environment"`. Here is the configuration manifest for the