POP vs IMAP vs SMTP

POP is protocol for downloading the files from ISP or email provider
SMTP is way of sending/uploading mail to email provider....
IMAP is also for downloading the files but it leaves a copy of message on the server so mail can be accessed from multiple machines.

All of them application layer protocol...

ISP POP is point of presence... Internet big picture is explained on this page... wonderful... http://navigators.com/internet_architecture.html