It uses its own optimized piece library and works with a variety of Lego CAD formats based around LDraw System as well as its own format. It is actually the system of choice over the alternative MLCad for most builders, although at present it cannot be used to construct parts from primitives.
It runs happily on both Linux, and Windows, and simple installers (binaries) can be downloaded, or it can be built from source.
It uses OpenGL and requires the GTK+ libraries on Linux.
With Windows, there is a shell extension (plug in for the Windows file explorer) which allows you to get information on drawings from a right-click context menu.
There is a relatively active community, and a couple of mailing lists where you can keep on top of its progress.