FreeCAD is a general purpose feature-based, parametric 3D modeler for CAD, MCAD, CAx, CAE and PLM, aimed directly at mechanical engineering and product design but also fits a wider range of uses in engineering, such as architecture or other engineering specialties. It is 100% Open Source (LGPL2+ license) and extremely modular, allowing for very advanced extension and customization.
FreeCAD is based on OpenCASCADE, a powerful geometry kernel, features an Open Inventor-compliant 3D scene representation model provided by the Coin 3D library, and a broad Python API. The interface is built with Qt. FreeCAD runs exactly the same way on Windows, Mac OSX, BSD and Linux platforms.
Precompiled (installable) packages are available for Windows and Mac on the Releases page.
On most Linux distributions, FreeCAD is directly installable from the software center application.
Other options are described at the wiki Download page.
Compiling FreeCAD requires installation of several libraries and their development files such as OpenCASCADe, Coin and Qt, listed in the pages below. Once this is done, FreeCAD can be simply compiled with cMake. On Windows, these libraries are bundled and offered by the FreeCAD team in a convenient package. On Linux, they are usually found in your distribution's repositories, and on Mac OSX and other platforms you will usually need to compile them yourself.
The pages below contain up-to-date build instructions:
Usage & Getting help
The FreeCAD wiki contains documentation on general FreeCAD usage, Python scripting, and development. These pages might help you get started:
The FreeCAD forum is also a great place to find help and solve specific problems you might encounter when learning to use FreeCAD.