From phpBB Development Wiki
What are extensions?
Extensions are the successor to MODs in phpBB 3.1. Extensions are different from MODs in that they do not make any file modifications to the core phpBB files. Instead, extensions are entirely contained in their own directories within the ./ext/ directory in the phpBB root.
First of, please take a look at the Developing Extensions wiki page. That page explains the basics of extensions.
Events and Listeners
Events and listeners were designed to get rid of the need to edit core phpBB files and are a main part of extensions. A detailed explanation can be found in the Event and Listeners category.
Migrations will be replacing UMIL in phpBB 3.1. Detailed explanations of how to use Migrations can be found at the Migrations wiki page. The Migrations/Extensions page explains how Migrations can be used in Extensions.
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