PartDesign Workbench
The 
PartDesign Workbench provides advanced tools for modeling complex solid parts. It is mostly focused on creating mechanical parts that can be manufactured and assembled into a finished product. Nevertheless, the created solids can be used in general for any other purpose, such as architectural design, finite element analysis, machining and 3D printing.
The PartDesign Workbench is intrinsically related to the Sketcher Workbench. The user normally creates a Sketch, then uses the PartDesign Pad tool to extrude it and create a basic solid, and then this solid is further modified.
While the 
Part Workbench is based on a constructive solid geometry (CSG) methodology for building shapes, the PartDesign Workbench uses a parametric, feature editing methodology, which means a basic solid is sequentially transformed by adding features on top until the final shape is obtained. See the feature editing page for a more complete explanation of this process, and then see Creating a simple part with PartDesign to get started with creating solids.
A more detailed discussion of Part workbench versus Part Design workbench can be found here: Part and Part Design.
The bodies created with PartDesign are often subject to the topological naming problem, which causes internal features to be renamed when the parametric operations are modified. This problem can be minimized by following the best practices described in the feature editing page and by taking advantage of datum objects as support for sketches and features.
PartDesign workbench icon
The Part Design tools are all located in the Part Design menu and the PartDesign toolbar that appear when you load the Part Design workbench.
These tools are, in fact, not part of the PartDesign Workbench. They belong to the Std Base system. They were developed in v0.17 with the intention that they would be useful to organize a model and create assemblies; as such, they are very useful when working with bodies created with this workbench.
Part: adds a new Part container in the active document and makes it active.
Group: adds a Group container in the active document, which allows organizing the objects in the tree view.
Create body: creates a Body object in the active document and makes it active.
Create sketch: creates a new sketch on a selected face or plane. If no face is selected while this tool is executed, the user is prompted to select a plane from the Tasks panel. The interface then switches to the Sketcher Workbench in sketch editing mode.
Edit sketch: edit the selected Sketch.
Map sketch to face: maps a sketch to a previously selected plane or a face of the active body.
Validate sketch: verifies the tolerance of different points and adjusts them.
Create a datum point: creates a datum point in the active body.
Create a datum line: creates a datum line in the active body.
Create a datum plane: creates a datum plane in the active body.
Create a local coordinate system: creates a local coordinate system attached to datum geometry in the active body.
Create a shape binder: creates a shape binder referencing geometry from a single parent object.
Create a sub-object(s) shape binder: creates a shape binder referencing geometry from one or more parent objects.
Create a clone: creates a clone of the selected body.
These are tools for creating base features or adding material to an existing solid body.
Pad: extrudes a solid from a selected sketch.
Revolution: creates a solid by revolving a sketch around an axis. The sketch must form a closed profile.
Additive loft: creates a solid by making a transition between two or more sketches.
Additive pipe: creates a solid by sweeping one or more sketches along an open or closed path.
Additive helix: creates a solid by sweeping a sketch along a helix.
Create an additive primitive: adds an additive primitive to the active body.
Additive box: creates an additive box.
Additive cylinder: creates an additive cylinder.
Additive sphere: creates an additive sphere.
Additive cone: creates an additive cone.
Additive ellipsoid: creates an additive ellipsoid.
Additive torus: creates an additive torus.
Additive prism: creates an additive prism.
Additive wedge: creates an additive wedge.
These are tools for subtracting material from an existing body.
Pocket: creates a pocket from a selected sketch.
Hole: creates a hole feature from a selected sketch. The sketch must contain one or multiple circles.
Groove: creates a groove by revolving a sketch around an axis.
Subtractive loft: creates a solid shape by making a transition between two or more sketches and subtracts it from the active body.
Subtractive pipe: creates a solid shape by sweeping one or more sketches along an open or closed path and subtracting it from the active body.
Subtractive helix: creates a solid shape by sweeping a sketch along a helix and subtracting it from the active body.
Create a subtractive primitive: adds a subtractive primitive to the active body.
Subtractive box: adds a subtractive box to the active body.
Subtractive cylinder: adds a subtractive cylinder to the active body.
Subtractive sphere: adds a subtractive sphere to the active body.
Subtractive cone: adds a subtractive cone to the active body.
Subtractive ellipsoid: adds a subtractive ellipsoid to the active body.
Subtractive torus: adds a subtractive torus to the active body.
Subtractive prism: adds a subtractive prism to the active body.
Subtractive wedge: adds a subtractive wedge to the active body.
These are tools for transforming existing features.
Mirrored: mirrors one or more features.
Linear Pattern: creates a linear pattern of one or more features.
Polar Pattern: creates a polar pattern of one or more features.
Create MultiTransform: creates a pattern by combining any of the transformations mentioned above, as well as the Scaled transformation.
Scaled: scales one or more features. This is not available as a separate transformation tool.
These tools apply a treatment to edges or faces.
Fillet: fillets (rounds) edges of the active body.
Chamfer: chamfers edges of the active body.
Draft: applies an angular draft to selected faces of the active body.
Thickness: creates a thick shell from the active body and opens the selected face.
Boolean operation: imports one or more Bodies or PartDesign Clones into the active body and applies a Boolean operation.
Some additional functionality found in the Part Design menu:
Migrate: migrates files created with older FreeCAD versions. If the file is purely PartDesign feature-based, migration should succeed. If the file contains mixed Part/Part Design/Draft objects, the conversion will most likely fail.
Sprocket: creates a sprocket profile that can be padded.
Involute gear: creates an involute gear profile that can be padded.
Shaft design wizard: Generates a shaft from a table of values and allows to analyze forces and moments. The shaft is made with a revolved sketch that can be edited.
Set tip: redefines the tip, which is the feature exposed outside of the Body.
Move object to other body: moves the selected sketch, datum geometry or feature to another Body.
Move object after other object: allows reordering of the Body tree by moving the selected sketch, datum geometry, or feature to another position in the list of features.
Appearance: determines appearance of the whole part (color transparency etc.).
Set colors: assigns colors to part faces.
Preferences: preferences available for PartDesign Tools.- Fine tuning: some extra parameters to fine-tune PartDesign behavior.
For some ideas of what can be achieved with Part Design tools, have a look at: PartDesign examples.
