A plugin (also known as an extension or addon) is a small bundle which is loaded to add functionality that is not already incorporated into the existing source code. They differ from scripts as they are designed to be native and provide more than the ability to return messages during execution. Plugins are for more experienced developers as a basic understanding of the Objective-C programming language is required.
Developers seeking to develop a plugin can read our quick introduction. The introduction includes a step-by-step video series. Afterwards, any questions not answered in the documentation can be answered by visiting #textual-dev on the freenode IRC Network.
For developers looking to make only simple scripts please read the Writing Scripts introduction.