Odoo is a suite of web based open source business apps.
The main Odoo Apps include an Open Source CRM, Website Builder, eCommerce, Warehouse Management, Project Management, Billing & Accounting, Point of Sale, Human Resources, Marketing, Manufacturing, Purchase Management, ...
Odoo Apps can be used as stand-alone applications, but they also integrate seamlessly so you get a full-featured Open Source ERP when you install several Apps.
For a standard installation please follow the Setup instructions from the documentation.
Then follow the developer tutorials
https://help.github.com/articles/configuring-a-remote-for-a-fork/
https://help.github.com/articles/syncing-a-fork/