Workbenches
FreeCAD, like many modern design applications such as Revit or CATIA, is based on the concept of Workbench. A workbench can be considered as a set of tools specially grouped for a certain task. In a traditional furniture workshop, you would have a work table for the person who works with wood, another one for the one who works with metal pieces, and maybe a third one for the guy who mounts all the pieces together.
In FreeCAD, the same concept applies. Tools are grouped into workbenches according to the tasks they are related to.
When you switch from one workbench to another, the tools available on the interface change. Toolbars, command bars and possibly other parts of the interface switch to the new workbench, but the contents of your scene doesn't change. You could, for example, start drawing 2D shapes with the Draft Workbench, then work further on them with the Part Workbench.
Note that sometimes a Workbench is referred to as a Module. However, Workbenches and Modules are different entities. A Module is any extension of FreeCAD, while a Workbench is a special type of Module with a GUI configuration (toolbars and menus).
Built-in workbenches
Current
The following workbenches are bundled with every FreeCAD installation:
Std Base: This is not really a workbench, but rather a category of 'standard' commands and tools that can be used in all workbenches.
The Arch Workbench for working with architectural elements.
The Draft Workbench contains 2D tools and basic 2D and 3D CAD operations.
The FEM Workbench provides Finite Element Analysis (FEA) workflow.
The Inspection Workbench is made to give you specific tools for examination of shapes. It is still under development.
The Mesh Workbench for working with triangulated meshes.
The OpenSCAD Workbench for interoperability with OpenSCAD and repairing constructive solid geometry (CSG) model history.
The Part Workbench for working with geometric primitives and boolean operations.
The Part Design Workbench for building Part shapes from sketches.
The Path Workbench is used to produce G-Code instructions. It is still under development.
The Points Workbench for working with point clouds.
The Reverse Engineering Workbench is intended to provide specific tools to convert shapes/solids/meshes into parametric FreeCAD-compatible features. It is still under development.
The Robot Workbench for studying robot movements.
The Sketcher Workbench for working with geometry-constrained sketches.
The Spreadsheet Workbench for creating and manipulating spreadsheet data.
The Start Center Workbench allows you to quickly jump to one of the most common workbenches.
The Surface Workbench provides tools to create and modify surfaces. It is similar to the Part Builder Face from edges option.
The TechDraw Workbench for producing technical drawings from 3D models. It is the successor of the Drawing Workbench.
The Test Framework Workbench is for debugging FreeCAD.
The Web Workbench provides you with a browser window instead of the 3D view within FreeCAD.
Obsolete
The following workbenches are no longer included after version 0.20:
The Drawing Workbench was used for producing technical drawings. The TechDraw Workbench is its more advanced replacement.
The Image Workbench was used for working with bitmap images. It functionality has been integrated in Std Base. See Std Import and Std ViewLoadImage.
The Raytracing Workbench was used for ray-tracing (rendering). The external Render Workbench should be used instead.
External workbenches
FreeCAD workbenches are easy to program in Python, there are therefore many people developing additional workbenches outside of the FreeCAD main development area.
The external workbenches page lists all that are known to this community. Most are easily installable from within FreeCAD, using the Addon Manager, found under menu Tools → Addon manager.