Addon

Introduction
In FreeCAD and in this documentation, an addon is any component that is not part of the base installation, but that can be added to the system by certain methods.

Different types
There are roughly three types of addons:
 * Macro: short snippet of Python code that provides a new tool or functionality in a single file ending with.
 * Module: a single Python source file, or a collection of Python files, that extends the software in some way. Modules don't necessarily define a graphical "workbench" but may provide a supporting feature, for example, a library that performs conversion of formats, or code that modifies the graphical interface.
 * Workbench: collections of Python files that provide related Gui Commands (tools) centered around a particular topic, for example, tools to design cabinets, or tools to work with architecture, or tools to design boats, etc. These workbenches usually define new toolbars where commands are placed as buttons.

Macros as installed under the user's directory, while modules and workbenches are under the  directory.

Macros usually start as a way to simplify or automate the task of drawing or editing a particular object. If many of these macros are collected inside a directory, and structure is provided to collect those tools, then the entire directory may be distributed as a workbench.

In other words, macros, modules, and workbenches are essentially the same thing, pieces of Python code that extend the base installation. Macros are usually short and focused on a single task, modules usually provide new functions or interfaces, and workbenches are collections of tools (buttons, menus) and graphical interfaces to perform related tasks.

If a workbench is sufficiently developed and is well documented, it may be included as one of the base workbenches in FreeCAD.

Installation
Starting from FreeCAD 0.17, the recommended way to install addons is with the Addon Manager.

However, manual installation is still possible.
 * How to install macros
 * Installing more workbenches