QWeb is Odoo's preferred reporting engine.  It was written by Antony Lesuisse, Odoo's CTO, in 2006 and is available in the following languages: Python, PHP, Ruby, C#, Java and Javascript.  

Odoo uses both the Python and Javascript versions of QWeb.  The Javascript version was ported and enhanced by Fabien Meghazi.

Documentation can be found on GitHub and in Odoo's technical documentation under:

• Building a Module - Reporting
• Building Interface Extensions - The QWeb Template Engine
• Views - QWeb
• Reference - QWeb
• Reference - QWeb Reports

Also available is a QWeb Video Presentation and the presentation slides from OpenDays 2014 by Simon Lejeune.