Ethel: I'm sorry, but that's wrong.
POP3 is the Post Office Protocol. A protocol is just a set of rules and regulations that defines how something should be done. the Post Office Protocol refers to how email should be sent from an email server to a client.
An email server is required to send and receive email. This is a computer setup to deal with email, using various protocols, one such being POP. Your email server may be your ISP, or it could be someone like Hotmail or Google (with Gmail). If someone (a user) wants to email you, they open their client application. This is a program that does the task of talking to the email server from the user's side. One such application is Outlook Express (and hence it's an email client, not a server).
When they send you an email, their client sends the email to their email server. This then looks to see who it's addressed to, and sends it on to your email server. When your email server gets it, it's stored on that computer.
Now, you come to your computer a bit later and open your email client (Outlook Express, or even gmail or hotmail in your web browser), and check your mail. If you're using the POP system, this means that you ask your server for all new mail, and it sends it to your computer (removing it from the server). You can then read or do whatever you want with it on your computer.
That's all POP is. Just a set of rules to specify how you are to receive email from a server. It means that email is moved from server to you, and not just kept on the server.