POP stands for Post Office Protocol, and it is one of the technologies used for that all-important medium of communication: email. Like many other computer-related things, email requires a special language for mail to be received or sent. POP is one of the technologies that allows email sent from anywhere in the world to arrive in a person's inbox.
When a person sends an email to a specific address, it is transmitted over the Internet and eventually lands on an email server. In order for an individual's personal computer to get that mail, it must follow a certain protocol. POP allows a computer to talk to the email server and then download all of the messages each time it connects.
One of the advantages of this technology is the ability to download all of the messages onto a local computer. This way, the user can view his messages on the go, without having a live Internet connection. Before a flight, a user could access all of his email and then view them via laptop in the plane when an Internet connection may be limited or unavailable.
Another benefit is for email servers with limited storage space. A small company may have only one small email server for its entire staff, for example. If each person periodically downloads all of his or her email to a local computer, it saves the company from having to invest in more storage space.
Of course, there are always downsides, and POP is no exception. The protocol is not the ideal choice for the increasingly mobile population with more than one computer. Once the email is downloaded off the email server, it resides permanently on the local computer unless it's sent somewhere else. An office worker accessing email in the office, for example, will not be able to then view email again at home, since all the email will be stored on the office machine.
Other solutions that address this concern include IMAP and Microsoft Exchange Server email accounts. These technologies allow all email to be read directly from a server, so all messages are readily available no matter what computer the person is using. The downside to this is that, if there is an Internet access disruption, then it will not be possible to access the email at all.