I assume you're talking about the Command Line, used in the terminal? If so, which distribution(s) are you wanting to know? Each distro shares some commands, and has their own commands.
Here's a good place to start:https://help.ubuntu.com/community/UsingTheTerminal
(For Ubuntu, but includes the majority of what you'll need)http://linux.org.mt/article/terminal
Again, if you let us know which distro you're running we can help further.