Appcelerator SDK is used to create applications or any third-party Ajax library / framework can be used too.
Tip: To make it clear, it is very similar to Adobe AIR but open source.
Titanium also comes with a simple command-line interface (CLI) for creating projects and packaging them into an executable.
The platform has a built-in database, direct file system access, desktop notifications & more.
Titanium apps. currently work in Microsoft Windows and Mac OSX. Linux support is in the roadmap.
More to follow about Titanium and some sample application / tutorial in coming posts.