What is the difference between inventory group_vars and playbook group_vars?

The following document

http://docs.ansible.com/ansible/playbooks_variables.html#variable-precedence-where-should-i-put-a-variable

states that inventory group_vars have lower precedence than playbook group_vars.

But I have just one group_vars directory in my installation. When I put anything into the group_vars directory, what is it? Is it an inventory group_var or an playbook group_var?

Depends on what they are adjacent to, inventory or play. If you only use one the distinction is mostly irrelevant.​