What are magic variables, and what do hostvars, group_names, groups and inventory_hostname contain?

• hostvars: the variables of every managed host, so one host can read another's.
• group_names: the groups the current host belongs to.
• groups: every group in the inventory and its hosts.
• inventory_hostname: the current host's name as written in the inventory.

Magic variables are set by Ansible itself. Their names are reserved: don't define your own variables with these names.

Two details worth knowing:

• hostvars contains a host's facts only once facts have been gathered for that host, in this play or an earlier play of the run. Before that, hostvars['db1']['ansible_facts'] is empty.
• inventory_hostname can differ from ansible_facts['hostname']. The inventory might call a host web1 (or an IP address, or an alias with ansible_host) while the machine calls itself srv-4711. Use inventory_hostname when you mean "this inventory entry", and the fact when you mean what the OS reports.

Go deeper:

• Special variables (official docs) — the full list of magic variables.
• Information about Ansible: magic variables (official docs) — examples with hostvars and groups.