See http://docs.ansible.com/playbooks_conditionals.html
There are a number of places you can provide a :ref:`host-spec` to limit which hosts something is applied to.
You must provide a host spec on an :ref:`ansible-command` ad-hoc command. 'Nuf said, we don't really use ad-hoc commands.
The --limit <hostspec> option can be provided on the
:ref:`ansible-command` and :ref:`ansible-playbook` command.
This does not say which hosts to apply the command to. It says do not apply the command to any host not in this hostspec, which is slightly different.
In a play, the hosts key is the hostspec to apply the play to.
Remember that a playbook may contain multiple plays. You can use this to apply different sets of tasks and roles to different hosts within a single playbook.
Use the hosts key to limit the hosts that the play's roles
and tasks will be applied to. Assume that the play might be invoked
without previous limiting, e.g. included from a playbook that applies
to more hosts, or used on a command line without giving a limit, perhaps
inadvertently.
To see what hosts would be affected by a playbook before you run it, you can do this:
ansible-playbook playbook.yml --list-hosts