When you request that a package is installed, and as a result other packages are installed to satisfy its dependencies, the dependencies are marked as being automatically installed, while the package you installed explicitly is marked as manually installed. Once an automatically installed package is no longer depended on by any manually installed package it is considered no longer needed and e.g.  apt-get or aptitude will at least suggest removing them.

       auto
           auto is used to mark a package as being automatically installed, which will cause the package to be removed when no more manually installed packages depend on this package.

       manual
           manual is used to mark a package as being manually installed, which will prevent the package from being automatically removed if no other packages depend on it.

       showauto
           showauto is used to print a list of automatically installed packages with each package on a new line. All automatically installed packages will be listed if no package is given. If packages are given only those which are automatically installed will be
shown.

       showmanual
           showmanual can be used in the same way as showauto except that it will print a list of manually installed packages instead.

from man apt