The official documentation is at: http://docs.alfresco.com
Alfresco Module Package (AMP) files are a standard way of providing extensions to the core Alfresco distribution. They work by modifying the Alfresco WAR deployment file. An AMP is installed by running the Module Management Tool. Each time an Alfresco WAR file is upgraded, any AMPs will need to be applied again.
Alternatively to SDK+Ant+MMT approach, Alfresco modules can now also be created, packaged and deployed using Apache Maven2 lifecycle management. This is an external contribution.