What is an inventory? Seriously? If you have to ask... An inventory is a collection of hosts organised into groups. With Ansible, that’s a poor answer though. There’s a lot more nuance to inventories than just a simple list.
Playbooks apply roles to a group in the inventory. Each group is referenced in the playbook as a named entity, but in actual use the playbook is applied to each server in the list. This mechanism separates the ‘what’ from the ‘where’ so that the roles in the playbook can be applied to many servers.
In database parlance, this establishes a many-to-many relationship between roles and inventory entries. Each server can have many roles, and each role can be applied to many servers.
Continuing with the Wordpress server example, the inventory file might look something like this:
It’s an ‘INI’ style format; a heading (the group name) and a simple list of hosts. More headers provide more groups, and more hosts equates to more servers.
There’s a whole bunch of other options available such as using multiple inventories, groups of groups, patterns for defining many servers, and dynamic inventories. It’s also possible to add server-specific variable in the inventory file.