The Part module deal with the CAD data structures.
At the moment the CAD capabilites of FreeCAD are very limited but it already supports a few basic CAD primitives like plane, box, cylinder, cone, sphere and torus. For all these primitives a user interface exists to create such elements. Moreover there are several additional primitives for which no user interface exists but that can be created over their Python interface. These primitives are circle and line. More primitives will follow.
To create primitives over the user interface switch to the Part design workbench and click on 'Create primitives...' menu item under the Part menu. This opens a dialog where your are able to define the type of primitive and define further properties depending on the type you have chosen. Then click on the 'Create' button to create such an element.