In AEM, the Externalizer is an OSGI service that allows you to programmatically transform a resource path (e.g. /path/to/my/page) into an external and absolute URL (for example, http://www.mycompany.com/path/to/my/page) by prefixing the path with a pre-configured DNS.
Because an instance can not know its externally visible URL if it is running behind a web layer, and because sometimes a link has to be created outside of the request scope, this service provides a central place to configure those external URLs and build them.
This page explains how to configure the Externalizer service and how to use it. For more details, please refer to the Javadocs.