Command line

Using the Linux command line may remember some of MS-DOS. For people who have used a unix-compliant operating system before (Mac OS X, for instance) the command line may look familiar.

One interesting site is LinuxCommand.org -- an interactive linux terminal clone, allowing you to practice your form through your web browser.

Command line guides

• tuXfiles - Linux command line tutorials for newbies
• A beginner's guide to the GNU/Linux command line
• Linux command line tools - Top 50 (good page, a reference with down to earth descriptions)
• 23 super-useful Linux command line tools: IT pros’ choices

Command references

• PixelBeat.org - Linux Command Line Tips
• Linux BASH Commands - MAN Pages
• GNU/Linux Command Line Tools Summary - well-seasoned database of commands

SSH

SSH, or Secure Shell is a network protocol allowing people to access terminals remotely. 

• Eleven SSH Tricks | Linux Journal