Terminal goes to the very heart of OSX. This is an application to be used with caution.

I suppose it would be difficult to really mess something up unless you tried very hard.

It's simply the application in which you write in code to have it take action directly on the system. You'd use it if you're having to deal with problems in the system.

For example, I used to rewrite some key maps when I had to run a certain program through X11 and the command key was switched with the control key.

Its uses vary widely depending on your situation.
I don't know about the syntax used to write lines of code, but I do know it's UNIX if that means anything to anyone. Every time I've used it I've simply copy-and-pasted code into it.

So I'm not very well educated on the specifics of terminal, but here is a blog that is.