One of the most important skills to have for navigating the command line in Linux is the ability to use manpages. The term "manpage", also sometimes spelled "man page", is short for "manual page". It refers to the online UNIX "manual", the traditional and most commonly available form of electronic documentation included with UNIX-based operating systems. They are often the subject of complaints for new Linux users who are used to GUI help systems and are not accustomed to using the keyboard to navigate through documentation, but the truth of the matter is that manpages are incredibly easy to use, and an indispensable resource.
http://www.builderau.com.au/program/linux/soa/Making_manpages_work_for_you/0,339028299,339272246,00.htm