execute on all hosts:

ansible-playbook -i inventory/file/path playbook/file/path

for all hosts:

ansible -i inventory/file/path all -m setup

for a single host:

ansible 192.168.1.20 -i inventory/file/path -m setup

redirect the output to a file:

ansible -i inventory/file/path all -m setup > system_facts.txt

these facts are stored in the ansible_facts variable and they are used, for example, in the when condition:

- name: install apache
  yum:
    name: httpd
    state: latest
  when: ansible_facts['os_family'] == 'RedHat'

README STILL IN PROGRESS...