Keepers have the job of guarding the three hoops. If they were to fail and the Quaffle went through one of them, the other team would be given 10 points. Although there are three hoops of 3 different sizes, this does not affect the amount of points they get for each hoop. 

Keepers used to be like chasers only given the privilege to guard the posts as well. However this has stopped because then it would be more easier to stop the other team from scoring. Keepers are usually well-built and athletic players and are usually chosen for their speed.