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.
Getting started with Odoo
For a standard installation please follow the Setup instructions from the documentation.
Then follow the developer tutorials