Part Module
The solid modelling capabilities of FreeCAD are based on the OpenCASCADE Technology (OCCT) kernel, a professional-grade CAD system that features advanced 3D geometry creation and manipulation. The 
Part Workbench is a layer sitting on top of the OCCT libraries, that gives the user access to OCCT geometric primitives and functions. Essentially all 2D and 3D drawing functions in every workbench (
Draft, 
Sketcher, 
PartDesign, etc.), are based on these functions exposed by the Part Workbench. Therefore, the Part Workbench is considered the core component of the modeling capabilities of FreeCAD.
A more detailed discussion of Part workbench versus Part Design workbench can be found here: Part and Part Design.
The objects created with the Part Workbench are relatively simple; they are intended to be used with boolean operations (unions and cuts) in order to build more complex shapes. This modeling paradigm is known as the constructive solid geometry (CSG) workflow, and it was the traditional methodology used in early CAD systems. On the other hand, the PartDesign Workbench provides a more modern workflow to constructing shapes: it uses a parametrically defined sketch, that is extruded to form a basic solid body, which is then modified by parametric transformations (feature editing), until the final object is obtained.
Part objects are more complex than mesh objects created with the Mesh Workbench, as they permit more advanced operations like coherent boolean operations, modifications history, and parametric behaviour.
Part workbench icon
The Part Workbench is the basic layer that exposes the OCCT drawing functions to all workbenches in FreeCAD.
The tools are located in the Part menu or the Measure menu.
These are tools for creating primitive objects.
Box: Creates a box.
Cylinder: Creates a cylinder.
Sphere: Creates a sphere.
Cone: Creates a cone.
Torus: Creates a torus.
Tube: Creates a tube.
Create primitives...: A tool to create one of the following primitives:
Plane: Creates a plane.
Box: Creates a box. This object can also be created with the 
Box tool.
Cylinder: Creates a cylinder. This object can also be created with the 
Cylinder tool.
Cone: Creates a cone. This object can also be created with the 
Cone tool.
Sphere: Creates a sphere. This object can also be created with the 
Sphere tool.
Ellipsoid: Creates a ellipsoid.
Torus: Creates a torus. This object can also be created with the 
Torus tool.
Prism: Creates a prism.
Wedge: Creates a wedge.
Helix: Creates a helix.
Spiral: Creates a spiral.
Circle: Creates a circular arc.
Ellipse: Creates an elliptical arc.
Point: Creates a point.
Line: Creates a line.
Regular polygon: Creates a regular polygon.
Shape builder: Creates shapes from various primitives.
These are tools for creating new and modifying existing objects.
Extrude: Extrudes planar faces.
Revolve: Creates a solid by revolving an object (not a solid) around an axis.
Mirror: Mirrors the selected object across a mirror plane.
Fillet: Fillets (rounds) edges of an object.
Chamfer: Chamfers edges of an object.
Make face from wires: Makes a face from a set of wires (contours).
Ruled Surface: Creates a ruled surface.
Loft: Lofts from one profile to another.
Sweep: Sweeps one or more profiles along a path.
Section: Creates a section by intersecting an object with a section plane.
Cross sections: Creates one or more cross-sections through an object.
Offset tools:
3D Offset: Constructs a parallel shape at a certain distance from an original.
2D Offset: Constructs a parallel wire at certain distance from an original, or enlarges/shrinks a planar face.
Thickness: Hollows out a solid.
Projection on surface: Projects a logo, text or any face, wire or edge onto a surface.
Attachment: Attaches an object to another object.
These tools perform boolean operations.
Compound Tools:
Boolean: Performs boolean operations on objects.
Cut: Cuts (subtracts) one object from another.
Fuse: Fuses (unions) two or more objects.
Common: Extracts the common (intersection) part of two objects.
Join features:
Connect: Connects interiors of walled objects.
Embed: Embeds a walled object into another walled object.
Cutout: Creates a cutout in a wall of an object for another walled object.
Splitting tools:
Boolean fragments: Creates all pieces obtained from Boolean operations.
Slice apart: Slices and splits an object by intersecting it with other objects.
Slice: Slices an object by intersecting it with other objects.
XOR: Removes space shared by an even number of objects.
Appearance: Determines the appearance of a whole object (color, transparency etc.).
Set colors: Assigns colors to individual faces of objects.
Preferences: Preferences available for Part Tools (the Part workbench also uses the PartDesign Preferences).
Import Export Preferences: Preferences available for importing from and exporting to different file formats.- Fine-tuning: Some extra parameters to fine-tune Part behavior.
See Part scripting
Make compound: Creates a compound from the selected objects.
Explode Compound: Splits up compounds.
Compound Filter: Extracts the individual pieces from compounds.
Measure Linear: Creates a linear measurement.
Measure Angular: Creates an angular measurement.
Measure Refresh: Updates all measurements.
Clear All: Clears all measurements.
Toggle All: Shows or hides all measurements.
Toggle 3D: Shows or hides 3D measurements.
Toggle Delta: Shows or hides delta measurements.
Import: Imports from .IGES, .STEP, or *.BREP files.
Export: Exports to .IGES, .STEP, or *.BREP files.
Box selection: Selects faces from a rectangular area.
Create shape from mesh: Creates a shape object from a mesh object.
Create points object from geometry: Creates a points object from a geometric object.
Convert to solid: Converts a shape object to a solid object.
Reverse shapes: Flips the normals of all faces of selected objects.
Create simple copy: Creates a simple copy of a selected object.
Create transformed copy: Creates a transformed copy of a selected object.
Create shape element copy: Creates a copy from an element (vertex, edge, face) of a selected object.
Refine shape: Cleans faces by removing unnecessary lines.
Check geometry: Checks the geometry of selected objects for errors.
Defeaturing: Removes features from an object.