What is a playbook, and what does a playbook look like? In terms of format, it’s just YAML with a few settings and the roles required for the target hosts.
In short, the playbook is what to do, the inventory is where to do it. As a practical example, this playbook to install a WordPress server (from https://github.com/ansible/ansible-examples/blob/master/wordpress-nginx/site.yml) illustrates how a set of roles is applied to a set of hosts.
- name: Install WordPress, MySQL, Nginx, and PHP-FPM
# remote_user: user
# sudo: yes
As we can see, it defines a display name for the playbook, the hosts it will be applied to, the remote user to use, and a toggle to control use of sudo.
It could run as root if you permit SSH connections as root (hint: you should never allow this), or as a regular user account with sudo rights.
Finally, the playbook defines roles for the server. These are the roles that this set of hosts needs. In this example, the common role is applied, along with mysql for the database, nginx for a web server, php-fpm for scripting, and the wordpress application.
Each role is distinct and atomic - it should be possible if the roles are designed properly to install the mysql database elsewhere on it’s own server for example.