Starterkit Project06 Question

Hi! I'm new to arduino and i'm testing all the projects that comes on the arduino book delivered with the starterkit.
i have a question.

Inside the code the book page 74 line 4 make reference to ledPin=13; so i asumed that i would connect a cable to this pin (13) on the arduine board, nevertheless the cable is connected to pin 8 .

and the project works fine , so my question is which is the real number of the pins on the board and wich is the correct way to call each pin number?

Thanks in advance