In Ansible, a playbook is is a YAML file containing the definition of how a server should look. In a playbook you define what actions Ansible should take to get the server in the state you want. Only what you define gets done.
This is a basic Ansible playbook that installs git on every host belonging to the web
group:
---
- name: Git installation
hosts: web
remote_user: root
tasks:
- name: Install Git
apt: name=git state=present