A student from Algebra II (4) has asked me this question using the Feedback.
Notes on Functions from our Moodle page is a place to start.
In terms of the test tomorrow, graphing piece-wise functions is the reference to "functions."
Take a look at the notes linked above. It will be important to know that f(x)=x-1 is the same as y=x-1.
Ask me in class for more information...