Let's consider a hypothetical class listing script.
A possible link URI:
class.php?display=students&class=4a
So $GET['display'] is "students", which might mean that class.php should query the database for the students in the class.
Which class? Well, $GET['class'] gives "4a", so presumably there is a class named "4a".
What if we wanted to know who is the teacher in charge of the class?
There might be:
class.php?display=teacher&class=4a