POP stands for Post Office Protocol. POP is a standard client/server protocol for sending and receiving email. The emails are received and held on your internet server until you pick it up with a client email program, like Outlook, Outlook Express, Netscape Messenger, etc. POP email programs are built into Netscape and Internet Explorer browsers (i.e. Microsoft Outlook Express).
Standing for Post Office Protocol, POP is a method of storing and retrieving emails.
Simply put, this is email that can be downloaded to an email program such as Outlook/Express, Entourage, MacMail, etc. This type of email allows you to compose and read emails offline, and allows you to save email on your own computer.