root
root is God. He controls all on a linux system. If you are the root user on a Linux system, it means that you can do anything required. You can enter other people's account, change files and all. You can also do maximum harm to your system, as there is no delete prevention etc. Hence it is better to work as a normal user than as root. Hence we generally make many user accounts, which have only limited powers. However only the root can do some jobs like installation of software etc.

The root is also can as the superuser or 'su'.  
   

Ctrl - alt -del
Also called the three-finger salute. It is used for rebooting. The moment you press these three keys, Linux automatically shuts down all systems and reboots irrespective of the programs open.  
   

Alt-F1, alt-f2...alt-f6
In Linux we have various terminals on each machine. You can open a new login on each terminal. Each terminal will behave like a new computer.
  

halt
Used for system shutdown. In ATX machines this will close all systems and turn off the CPU.
   

ls
The command most often used in Linux. 'ls' provides a directory and file listing in the given directory. There are various parameters such as 'ls -al' which provides a detailed version of stuff in that folder. For further help type 'man ls'. It is equivalent to the 'dir' command in dos.
  

cd (name) - change directory
mkdir (name) - make directory
rmdir (name) - remove directory, only possible if the directory contains only files. 
rm (filename) - removes the file from the folder. Can be done only if you have write permissions (We shall discuss permissions later)
   

startx
Starts X-Windows along with the Window - manager you have chosen.
  

man (command)
The most important command for newbies. References the manual page for that command. The manual page gives detailed information about each command, its uses, parameters etc. However, these manual pages are written by programmers themselves, and hence tend to be a little complicated. Hence I am here to help you out.
   

kill
The kill command is used to terminate processes that can't be stopped by other means. This happens very rarely.
   

pico filename
Invokes the text editor 'pico' and opens the given file. If the file is not present, then a new one will be created.
   

useradd (username)
This can be carried out only when logged in as root. It will create a new user by the given name.
  

passwd (username)
Changes the password for the given user. You can change the passwd for another user only if you are root. Change your own password by typing only passwd.