Where can I find this app, and where can I find instructions on how to use it?
My OS X is using the Dutch language settings.
When your system is
Your shell is usually
Be aware that you can seriously mess up or destroy your system, or delete all of your data if you don't know what you're doing in Terminal!
Terminal is under the Utilities folder, which is under Applications. You launch it, then it presents you with a command line. If you're not certain about what to do with it, you can cause damage to your OS. It all depends on what you're looking to do with it.