Welcome to the DD4T wiki!
DD4T is a framework on top of a CMS called SDL Tridion Sites. It helps you to develop an MVC web application based on the content you manage in Tridion.
At its core DD4T consists of a set of template building blocks to publish your pages and component presentations to the Tridion Content Data Store as JSON, and a set of .NET libraries which you use in your MVC web application and which retrieve the pages and component presentations from the Tridion Content Data Store (a.k.a. Broker database) and convert them to objects ('ViewModels').
DD4T exists for both .NET and Java. The template building blocks for both flavors are identical.
If you want to contribute to DD4T, you can do it in one of the following ways: