What is a package maintainer ?

Every linux package has a maintainer. But what does he do ? what is his job ? does he simply compile a new version of the software each time a new version is released by the original authors ? is he responsible for testing the software for that distribution ? does he make some changes to the original source code to adapt the package to the distro's peculiarities (installing files in different folders) ? what specific skills does one has to have to be a package maintainer ? does he have to be a master of config and make for the most part ? what's the toolchain like ? what does one has to learn to be a good package maintainer ?

If you are a package maintainer and would like so share your experience that would be much appreciated.