The Difference between GAT and GAX
When I first started playing with Software Factories, I was kind of confused by these two things called GAX and GAT and why I needed both to get started
Luckily, I got to find out about them on my recent visit to the Patterns & Practices team.

It turns out you don't
need both at all. D'oh.
Guidance Automation eXtensions
Visual Studio is extremely extensible. And it is this extensiblity that the P&P team leverage to create their software factories. However, Visual Studio extensibility is also nothing short of rocket science on occasions, which is why they decided to wrap the necessary extension points they needed with an add-in that exposed a much easier to use API. GAX was born.
Guidance Automation Toolkit
All the Software Factories, from the
Smart Client Software Factory to the
Web Services Software Factory are built atop the GAX. Given that these Software Factories, or
Guidance Packages are themselves full of repeatable patterns, it makes sense that you have a Software Factory Factory. Right? Well that's exactly what GAT, the Guidance Automation Toolkit, is and as such it's also built atop of the GAX.
Easy peasy. Stay tuned for more on factories.