Freestyle is an edge/line-based non-photorealistic (NPR) rendering engine. It relies on mesh data and Z-depth information to draw lines on selected edge types. Various line styles can be added to produce artistic ("hand drawn", "painted", etc.) or technical (hard line) looks.
Freestyle can generate a powerful diversity of line styles and results. There are currently, two ways to define the way lines look; the first uses a series of parameter to create a Line Style. This mode allows intuitive editing of features such as dotted lines and easy setup of multiple line types and edge definitions. On top of all of that, with line style modifiers, the sky is the limit!
The second method of generating lines is by using Python Scripting. This method is much more advanced but Blender includes many pre-scripted styles such as Japanese big brush, cartoon, blueprint, and thickness-with-depth.
The Big Picture
Activate Freestyle by thecheckbox.
Freestyle settings are located in the View Layer properties.
One view layer can only have one view map. A view map holds the edge detection settings (Crease Angle, Culling toggle, Face Smoothness toggle, Material Boundaries toggle, Sphere Radius, and Kr Derivative Epsilon advanced options).
A view map can have multiple Line Sets.
A line set controls which line types and selections will be rendered, from lines based on your scene.
Each line set uses one line style (which can be shared between multiple Line Sets).
A line style tells Freestyle how to render the linked Line Sets in terms of color, alpha, thickness and other aspects.
Highly memory demanding: All mesh objects in a view layer are loaded at once.
Only faced mesh objects are supported.
No edges at face intersections are detected yet.
Freestyle rendering results do not have any Z depth information.
Panoramic cameras are not supported.