The Difference Between Aorist and Perfectum ???

Hello everyone,

A few days ago at school we started with the second  section in our greek book. We talked about the aorist...  our teacher  explaned the difference between the imperfect and the aorist... but I want to know what the difference is between the aorist and the perfectum...

Yes, our teacher isn't really clear about those things... :S

Thanks for your help :D