man – format and display the on-line manual pages

man 1 expand
man -a open
man bash | col -b > bash.txt
man -t bash > bash.ps
man -t bash | ps2pdf - > bash.pdf

whatis – search the whatis database for complete words.

whatis open

apropos – search the whatis database for strings

apropos "create a file"
man -k  "create a file"

whereis – locate the binary, source, and manual page files for a command

whereis expand

which – shows the full path of (shell) commands.

which ls

type – indicate how each name would be interpreted if used as a command name

$ type expand
expand is /usr/bin/expand

$ type -a ls
ls is aliased to `/bin/ls $LS_OPTIONS'
ls is /usr/bin/ls
ls is /bin/ls

$ type -t ls
alias

$ type -t type
builtin

info – read Info documents

info bash
info info
info gcc
info libc
info gdb
info emacs
info "(standards)User Interfaces"
info coreutils 'printenv invocation'

Document directoris

/usr/doc/
/usr/doc/Linux-FAQs/
/usr/doc/Linux-HOWTOs