To read the "man page" (manual page) for a command such as "ls",
try "man ls".
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Want to find a man page on something but you are not completely sure
of the command? Try "apropos" or "man -k", followed by a keyword.
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The documentation to many commands is accessible using "info".  Be sure
to hit Control-H for help in the info system.
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Check the /usr/doc directory for documentation on various packages installed
on this system.
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Check the /usr/doc/HOWTO and /usr/doc/HOWTO/mini directories for
documentation on how to do specific things in Linux.
Check the /usr/doc/LDP directory for Linux Documentation Project books.

The latest versions of all this documentation may be found online at
  http://www.linuxdoc.org/
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Try using the commands "more", "less", or "most" to look at files.  (Use the
space bar to page forward, and the "q" key to quit.)  "cat" is good for
looking at small files.
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When in text mode, use Alt-F1 through Alt-F10 to switch "virtual consoles".
This way you can have multiple login sessions at once, and can quickly
switch between them.  In graphics mode, use Shift-Alt instead of just Alt.
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Use the "ls" command to see what files are in the current directory.
Add the "-a" option ("ls -a") to include hidden files (whose names begin
with a period).
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Use the "pwd" command to see what directory you are currently sitting in.
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Often the best way to look at an HTML file is with "lynx",
e.g. "lynx index.html".
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Files whose names end in ".gz" are compressed with gzip.  You can use
"gunzip" to change them to uncompressed files, or try using "zmore" or
"zless" to view the text within, without using gunzip first.
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Files whose names end in ".tar.gz" are called "tarballs".  They are similar
to zipped files, in that they are compressed collections of files.
The easiest way to extract the contents of a file called "filename.tar.gz"
is the command "tar xzf filename.tar.gz".
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Use "rm" to remove (delete) a file, and "rmdir" to remove an empty directory.
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Use "mkdir" to make a new directory, and "cd" to change the current directory.
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Use "cp" to copy a file, and "mv" to move or rename it.