DXA, what is it?
DXA is a reference implementation from SDL to use with SDL Web (formerly known as SDL Tridion) content management system. It uses DD4T and implements best practices on how to setup a site in SDL Web.
Great, but what is it for?
DXA stands for Digital eXperience Accelerator and is developed/supported by SDL. Its goal is to speed up CDS implementations by standardizing solutions and showing best-practices. DXA is an Open Source project which utilizes either .NET MVC or Spring MVC to provide dynamic content delivery on the CDS, depending of your technology stack.
DXA is open sourced on github and DXA documentation is available online.
Works best for new implementations.
The CMS installer adds a default blueprint structure to separate concerns. Each publication contains the necessary building blocks required for content assembly: structure groups, schemas, folders, components, and pages.
When you publish you get the sample site that showcases the DXA’s "out of the box" functionalities with dummy content and white label HTML design. This could be the starting point for further development.
What are the benefits?
Depends on how you look at it. There are benefits for business people (a.k.a. as the client) and benefits for developers. Also, there are some things to think about if DXA really supports your case. I explain more on this topic on my employer blog: http://blog.tahzoo.com/sdl-dxa-what-is-it-and-why-should-i-care/