The group_vars directory can be used to create unique variables for different groups of managed nodes (e.g. target systems). Or, if you have groups of managed nodes in your default hosts file or your own inventory file, you could just as well create unique variables for different groups of managed nodes in your default hosts file or your own inventory file.

Let's say you've a group of Linux servers and a group of Windows servers, like this, and the "foo" variable should contain a value of "Hello" for Linux servers, and a value of "World" for Windows servers.

[linux]
server1.example.com
[windows]
server2.example.com

Create the group_vars directory and the linux.yml and windows.yml files. The name of the YAML files must match the managed node group name.

mkdir /etc/ansible/group_vars
touch /etc/ansible/group_vars/linux.yml
touch /etc/ansible/group_vars/windows.yml

Use Hello in linux.yml.

foo: Hello

Use World in windows.yml.

foo: World

The debug module can be used to print the value in the "foo" variable. Notice in this example that the playbook will be run against the "linux" hosts.

---
- hosts: linux
  tasks:
    - debug:
        var: foo

In this example, the "foo" variable will contain a value of "Hello".

TASK [debug]
ok: [server1.example.com] => {
    "msg": "Hello"
}