Lookup plugins retrieve data from outside sources such as files, databases, key/value stores, APIs, and other services. Like all templating, lookups execute and are evaluated on the Ansible control machine. Ansible makes the data returned by a lookup plugin available using the standard templating system. Before Ansible 2.5, lookups were mostly used indirectly in ``with_<lookup>`` constructs for looping. Starting with Ansible 2.5, lookups are used more explicitly as part of Jinja2 expressions fed into the ``loop`` keyword.
For more details and a list of lookup plugins in ansible-base, see :ref:`plugins_lookup`. You may also find lookup plugins in collections. You can review a list of lookup plugins installed on your control machine with the command ``ansible-doc -l -t lookup``.