An addin is a generic extension to World of Goo that can be installed by GooTool. There are currently two types of addin:

Mod

A generic addin that extends World of Goo, generally by modifying existing functionality or providing new functionality such as new ball types.

Level

An addin that specifically creates a new level within World of Goo. Since modding isn't officially supported yet, new levels are added to Chapter 5.