Application Documentation
On your system, the /usr/share/doc directory contains documentation files installed by each application. Within each directory, you can usually find HOW-TO, README, and INSTALL documents for that application.
The Man Pages
You can also access the Man pages, which are manuals for Linux commands available from the command line interface, using the man command. Enter man with the command for which you want information. The following example asks for information on the ls command:
$ man ls
Pressing the SPACEBAR key advances you to the next page. Pressing the B key moves you back a page. When you finish, press the Q key to quit the Man utility and return to the command line. You activate a search by pressing either the slash (/) or question mark (?). The / searches forward; the ? searches backward. When you press the /, a line opens at the bottom of your screen, and you then enter a word to search for. Press ENTER to activate the search. You can repeat the same search by pressing the N key. You needn’t reenter the pattern.
The Info Pages
Online documentation for GNU applications, such as the GNU C and C++ compiler (gcc) and the Emacs editor, also exist as info pages accessible from the GNOME and KDE Help Centers. You can also access this documentation by entering the command info. This brings up a special screen listing different GNU applications. The info interface has its own set of commands. You can learn more about it by entering info info. Typing m opens a line at the bottom of the screen where you can enter the first few letters of the application. Pressing ENTER brings up the info file on that application.