Role defaults are variables defined as defaults within roles.

$ cat roles/myrole/defaults/main.yml
a_variable_name: myvalue

Inventory file group variables are group variables that are defined in an inventory file.

[atlanta]
host1
host2

[atlanta:vars]
ntp_server=ntp.atlanta.example.com
proxy=proxy.atlanta.example.com

Inventory group_vars/all variables are defined within an all group variables file found within the group_vars directory tree adjacent to the current inventory file.

.
├── group_vars
│   └── all
└── inventory

[user@host ~]# cat group_vars/all
a_variable_name: myvalue

Playbook group_vars/all variables are defined within an all group variables file found within the group_vars directory tree adjacent to the current playbook.

.
├── group_vars
│   └── all
└── myplaybook.yml

[user@host ~]# cat group_vars/all
a_variable_name: myvalue

Inventory group_vars/* variables are defined with a particular, group variables file other than all within the group_vars directory tree adjacent to the current inventory file.

.
├── group_vars
│   └── some-group
└── inventory

[user@host ~]# cat group_vars/some-group
a_variable_name: myvalue

Playbook group_vars/* variables are defined with a particular group variables file other than all within the group_vars directory tree adjacent to the current inventory file.

.
├── group_vars
│   └── some-group
└── myplaybook.yml

[user@host ~]# cat group_vars/some-group
a_variable_name: myvalue

Inventory file host variables are host variables defined in an inventory file.

[atlanta]
host1 http_port=80 maxRequestsPerChild=808
host2 http_port=303 maxRequestsPerChild=909

Inventory host_vars/* variables are variables that are defined in a host-specific variables file under the host_vars directory that is adjacent to the current inventory file.

.
├── host_vars
│   └── some-host
└── inventory

[user@host ~]# cat host_vars/some-host
a_variable_name: myvalue

Playbook host_vars/* variables are variables that are defined in a host-specific variables file under the host_vars directory that is adjacent to the current playbook file.

.
├── host_vars
│   └── some-host
└── myplaybook.yml

[user@host ~]# cat host_vars/some-host
a_variable_name: myvalue

Host facts are variables associated with a host and are defined in a particular way (as facts).

- set_fact:
    one_fact: foo
    other_fact: bar

Play variables are defined in the vars block that appears at the top of a play.

---
- name: this is a play
  hosts: groupname
  vars:
    foo: foovalue
    bar: barvalue

Play vars_prompt variables are defined in the vars_prompt block that appears at the top of a play.

---
- hosts: all
  remote_user: root

  vars:
    from: "camelot"

  vars_prompt:
    - name: "name"
      prompt: "what is your name?"
    - name: "quest"
      prompt: "what is your quest?"
    - name: "favcolor"
      prompt: "what is your favorite color?"

Play vars_files variables are defined in a file that is loaded by virtue of having its file path listed in the vars_files block that appears at the top of a play.

$ cat vars/example.yml
foovar: fooval
barvar: barval

$ cat myplaybook.yml
---
- name: my playbook
  hosts: myhosts
  vars_files:
    - vars/example.yml

Role variables are defined in role/vars/main.yml.

$ cat roles/myrole/vars/main.yml
foovar: fooval
barvar: barval

Block variables are defined under the vars heading within a block of Ansible statements that are logically grouped using the block module.

- block:
  - debug:
      var: var_for_block

  vars:
    var_for_block: "value for var_for_block"

Role parameters are defined under the vars heading that appears as an argument in conjunction with include_role.

---
- hosts: webservers
  tasks:
  - include_role:
      name: foo_app_instance
    vars:
      dir: '/opt/a'
      app_port: 5000

Include parameters are variables defined in conjunction with an include statement.

---
- name: This is an example play
  hosts: agroup
  vars:
    foo: bar

- include: amazon.yml
  vars:
    application: FooServer
    instance_type: t2.micro
    instance_count: 1
- include: amazon.yml
  vars:
    application: BarServer
    instance_type: t2.micro
    instance_count: 1

Extra variables (extra vars) are defined at the command line using the --extra-vars (-e) option.

$ ansible-playbook -e "version=1.23.45 another_variable=foobarino" myplaybook.yml