Short for Manual Page, a manpage is just that. A manpage should give a description of a CommandLine options, configuration file syntax,etc and a few examples.
The man-db package includes three tools for finding information and/or documentation about your Linux system: man, apropos and whatis.
The man system formats and displays on-line manual pages about commands or functions on your system.To view a ManPage, one simply types "man foo". The ManPage is formatted and displayed by the $PAGER.
Apropos searches the whatis database (containing short descriptions of system commands) for a string.
Whatis searches its own database for a complete word.
The manpages package should be installed on your system because it is the primary way for find documentation on a Linux system.
The man pages are stored in /usr/share/man.
Translations, locales
You can install translated pages in your language (see packages like manpages-fr ).
Locales.
Xman
The x11-apps package contains xman, a man page browser for the XWindow system. (Gnome's help system lets you search and read manpages too).
See also
http://manpages.debian.net - an online repository of the Debian Manpages.
http://www.debian.org/doc/debian-policy/ - Debian Policy's Documentation chapter.