3D Brush is main tool in Polybrush.

You can open window with brush presets by F5 hotkey or button at Brush panel (brush library is described more detail in next lesson)

3D Brush is very flexible tool which can be used for modeling and for sculpting.

I.e. there are two types of brushes - 3D stroke brushes (which creates new 3d meshes) and Sculpting Tools (which changes already existed objects in scene). Brush type can be selected at top of Brush window.

Most primitive brush that selected in Polybrush by default is simple round 3Dstroke-brush.

3DStroke brushes consists of one or few objects which stretches or coping by brush trajectory. 

You can look to objects inside brushes at "brush form" tab. Here you can also find buttons to replace object (from file or scene) or save it. At top of this tab placed buttons to switch between few forms in one brush.

You can choose how the object will be used. Stretch or copy. If the object is very simple, you can use it as a section. Use spacing slider to define difference between sections/copies.

If object used as section you can change form directly via UI. Use different mouse buttons to move/create/delete vertices.

For more complicated changes use modificators. They described more detailed in separate lesson. You can use modificators for one object inside brush or for all at once (by using "Parameters" stack). Modificator values can be linked to pen properties (pressure or speed for example).

You can create any necessary brush this way. For example, add "hue" modificator and link brush color to pen pressure.

In sculpting brushes modificators are using to change object on which you draw.

So because any object can be loaded to brush and modificators can be combined in any amount and order it makes really unlimited possibilities to create user brushes.