Definition

In Debian, maintainers are people who work on uploaded packages. There are :

Debian Developers :: Debian Project members who have gone through the Debian New Maintainers process and had their application accepted are called Debian Developers

Debian Maintainers (DM) :: Debian Maintainer is an official status of people, who are allowed to upload specific packages (with a DM field set). They don't need to request for sponsorship.

Sponsored maintainers :: You can be a sponsored maintainer. You only need a sponsor.

Disambiguation

Here is a list of expressions which use the word "maintainer" :

Debian New Maintainer :: The Debian New Maintainers process, is the process of becoming an official Debian Developer (DD).

Debian New Maintainers' Guide :: The Debian New Maintainers' Guide is the first document to read if you want to work with packages.