FreeCAD is a general-purpose parametric 3D computer-aided design (CAD) modeling application, made primarily to design real-life objects. Parametric modeling is a type of modeling where the shapes of the 3D objects you design are controlled by parameters. For example, the shape of a brick might be controlled by three parameters: height, width and length. These parameters are part of the object and may be modified at any time, even after the object has been created. As well as simple values, an object can have other objects as parameters. For example, you could have an object that takes your brick as input and creates a column from it. You could think of a parametric object as a small program that creates geometry from parameters.
FreeCAD is aimed directly at mechanical engineering and product design, but it is not designed for making a particular kind of object. Instead, it permits users to produce models of objects of widely differing sizes for many purposes, from small electronic components to 3D-printable pieces all the way up to buildings. Each of these tasks has different dedicated sets of tools and workflows available.
FreeCAD is also multiplatform (it runs exactly the same way on Windows, Mac OS, Linux and other platforms), and it is open source. FreeCAD benefits from the contributions and accumulated experience of the large community of open source programmers, enthusiasts and users worldwide. FreeCAD is essentially an application built by the people who use it, instead of being made by a company trying to sell you a product. Open source also means that FreeCAD is free, not only to use, but also to distribute, copy, modify, or even sell.
Internally, FreeCAD leverages several other open source components, and FreeCAD itself can be used as a component in other applications. It possesses many features that have become standard in the open source world, such as supporting a wide range of file formats, and being scriptable, customizable and modifiable.
FreeCAD is built using a modular software architecture which makes it easy to provide additional functionality without modifying the core system. Modules are loaded only when actually used, allowing additional modules to be added to an existing installation or unwanted ones to be removed.
FreeCAD makes extensive use of a built-in Python scripting framework. The interpreter can run complex scripts as well as single commands, affecting almost any aspect of the FreeCAD application; entire modules can be programmed completely in Python.
The official website of FreeCAD is at http://www.freecadweb.org
About the FreeCAD project
The FreeCAD project was started in 2001, as described in its history page.
FreeCAD is maintained and developed by a community of enthusiastic developers and users (see the contributors page). They work on FreeCAD voluntarily, in their free time. They cannot guarantee that FreeCAD contains or will contain everything you might wish, but they will usually do their best! The community gathers on the FreeCAD forum, where most of the ideas and decisions are discussed. Feel free to join us there!