Maintainer
A Maintainer is someone who maintains a Debian package (packaging, updating, tracking and fixing bugs, etc).
Debian Maintainer

Debian Maintainer is an official status of people, who are not (yet) Debian Developers, but that are allowed to upload packages, that have a DM field set.
More details at DebianMaintainer.

Debian New Maintainer

The Debian New Maintainer process, is the process of becoming an official Debian Developer (DD).
More details at http://www.debian.org/devel/join/newmaint.