Beginning with phpBB 3.1, database updating is performed using Migrations.

Migrations was built because of the difficulties of distributed development and maintaining an updated database setup across all developers.

RFC Topic if you’re interested in the reasons: http://area51.phpbb.com/phpBB/viewtopic.php?t=41337

What do Migrations mean for me?

As a user

Migrations do not affect using phpBB, only writing code for it, so Administrators do not have to do anything differently.

Database updates between versions of phpBB and Extension updates will be safer and, in the unlikely event something happens during database changes, much easier to debug and correct.

As an Extension author

Functionality (similar to UMIL for phpBB 3.0) built into phpBB for easier management of database changes between versions and helpful tools to assist making database changes.

As a Developer

Easier collaboration with others, less time spent managing database changes between different branches and pull requests.

How do I use Migrations?

Getting started with Migrations

How to create a basic Migration file Getting started

Migration Helpers

How to use Migration helpers to perform basic database changes Tools