pop

"POP" is pretty simple; that's an acronym for "Post Office Protocol". A communications "protocol" is just the language used between your email program, a POP client, and your ISP's mail - or POP - server.
"3" is even more boring. This is version 3 of the POP protocol. It underwent a few revisions before it became what it is today.
To configure a POP account you need three pieces of information:
The name of your ISP's mail server that holds your email. Typically it's something like "mail.example.com".
The name of the account you were assigned by your ISP. This may or may not be your email name, or something like it, or something completely unrelated.
The password to your account.
That's it. With that properly configured, you can download the email that your ISP has been collecting on your behalf.
Sending mail uses a different protocol, SMTP, which stands for Simple Mail Transfer Protocol. Again, another language used between your email program, an SMTP client, and the SMTP server to which you will send your email.
Typically your SMTP server will be the same as your POP3 server, though that's not always the case. If so, it doesn't really imply that the two are related, just that the same machine is acting as a server for both protocols.
Like POP3, the SMTP server may require you to log in first; often with the same account information that the POP3 server used. (If it doesn't require you to authenticate somehow, it's called an "open relay" and may be a major contributor to internet spam.)
So to configure your outgoing mail, you'll specify the name of the outgoing server, and possibly the login information it will use.
And finally, note that all of this really applies only to email programs that you run on your own computer, like Outlook, Eudora and others. Web-based email, such as Hotmail, Yahoo and the like, simply display the email directly from their servers in your web browser - no configuration needed, other than logging in.