What are some useful Terminal commands?
 

I already know sudo, and I already learned that it's case sensitive...
I know CD, Apt-Get, and Dir, but besides that I know mostly Command Prompt commands and that's not many, and they seem to differ from Terminal. What are some useful Terminal commands that I should know how to use in times or need? Could you also explain them?

I would google for "Linux Cheat Sheet" or "Linux Quick Reference". You'll get several results for command-line tricks :) one I like is files.fosswire.com/2007/08/fwunixref.pdf

Here's an awesome site with command line tips and tricks: http://www.commandlinefu.com/commands/browse

Terminal commands tend to be one of two.

One is a built in command that the terminal or shell may have. They can't usually be found in any search of files.

The other is any command that can be run, and that can be a lot of commands. Even normal gui type applications may have a command line.

The usual first spot is /bin for commands. See them. Use ls or ls -l. In fact get used to using man ls or man some command.

Also help may be found by ls --help for example.

When you are at /bin try each file for man (filename) and see if you get a help file.

q to exit man page.