YAML is the syntax used for Ansible playbooks and some other files. The official YAML documentation contains the full specifications of the syntax. Here is a compact version as it pertains to the most common usage for Ansible:

• A YAML file starts with three dashes (---)
• Whitespace indentation is used to denote structures when they are lined up, just like Python
• Comments begin with the hash (#) sign
• List members are denoted by a leading hyphen (-), with one member per line
• Lists can also be denoted via square brackets ([]), with elements separated by a comma (,)
• Dictionaries are denoted by key: value pairs, with a colon for separation
• Dictionaries can be denoted by curly braces, with elements separated by a comma (,)
• Strings can be unquoted, but can also be enclosed in double or single quotes

As you can see, YAML maps well into JSON and Python datatypes. If I were to rewrite df_playbook.yml into df_playbook.json, this is what it would look like:

        [
          {
            "hosts": "192.168.199.170",
            "tasks": [
            "name": "check disk usage",
            "shell": "df > df_temp.txt"
           ]
          }
        ]

This is obviously not a valid playbook, but serves as an aid in helping to understand the YAML formats while using the JSON format as a comparison. Most of the time, comments (#), lists (-), and dictionaries (key: value) are what you will see in a playbook.