PayPal: Instant Payment Notification

Hello there, Has anybody worked with PayPal's Instant Payment Notification? How exactly does it work -- will the payer ever go to PayPal's site itself (and leave my site for a sec) or will all contact with PayPal's servers be handled by my script? I have to write a system that enables payments via this service, but I'm low on documentation and this service is a new one for me. An overview of the process and pointers would be great! Thanks, Lauri -- Tharapita Creations [server-client web applications] lauri.vain@tharapita.com Mobile: +372 53 410 610