IMAP and POP email.
Which is better?
There are two email protocols that are in common use today. They are POP and IMAP. POP stands for Post Office Protocol and IMAP stands for Internet Message Access Protocol. The difference between the two is that POP email allows the messages to exist only on one computer at a time and IMAP allows the mail to exist on more than one computer at any one time. That is an oversimplification, but it gets to the core of the two mail protocols.
The POP email protocol is older than IMAP email. The basic way POP is used is to collect mail on one computer (the mail server) and deliver it to another computer (the user's local machine). By handling email messages in this way, it frees up server space and gives you the confidence that there is only one copy of the email. There are not various versions of each message on other machines. This is the "download and delete" aspect of POP email. Your computer connects to the POP email server and downloads a copy of everything on the server and then deletes the original messages once they are all delivered to you. There is actually a way to leave all the mail on the mail server and download the email to various computers. However, this goes against the way most local Internet providers want you to work.
This historically worked well as long as you only ever needed to access your mail on one machine at a time. If you worked in an office and used one machine to check and respond to email, you may have never run into the limitations of POP email. However, using POP email, there is no easy way to check and respond to mail on different computers without using a web interface. When you use the web interface exclusively, you do not download and delete the messages like the protocol was designed to do.
The biggest drawback with POP email is that the email messages you write from a mail client only exist on the machine they were written from. You can set your desktop computer at work to download and delete your email from the server while configuring your notebook to only download the messages. This allows you to get mail on both computers. However, anything you write on one computer cannot be seen on the other computer.
POP was designed in a time when webserver storage space was much more expensive than it is today. It was a necessity at Internet service providers (ISP) to limit the amount of space someone could have to store their mail. That is less of a limitation today. Another aspect of POP email is that it assumes the client will only be connected to the Internet periodically. Now that people are more and more connected to the Internet constantly (instead of using dial-up connections), checking mail no longer needs to be a specific thing you do once or twice a day.
The problem with using multiple computers to receive and respond to email was solved with the introduction of the IMAP email protocol. This mail protocol is designed to keep all the mail on a central server and have multiple machines make copies of the messages. When a computer checks the mail server, the messages are all synchronized from the server to the client and vice versa. If there are any changes to a message status (deleted, forwarded or moved to another folder) the email client will alert the IMAP email server of the changes. The server will then inform any other computers of the changes whenever they connect to the email server.
With this system you can think of the mail as still existing in only one place, but that the individual computers "mirror" what is on the server. The computers can then push changes back to the server, and the server propogates the changes out to the other computers when they connect. This allows all of your computers which access your IMAP server to have a mirror image of all the messages sent and received by all of your computers.
Which is better, POP or IMAP email?
It should be obvious which is the better option for the end user. However, what is better for the user is not always better for the person or company running the mail server. IMAP email systems require more storage space than POP email because any message that is not deleted on a client machine is continuously stored on the server. ISPs still would rather you use POP instead of IMAP because of this. However, companies which have their own email servers usually choose to have an IMAP server as a benefit to their workers.
A company providing email storage to a few hundred workers is better able to offer the extra storage that IMAP requires per user than an ISP which may be serving tens of thousands of users. At some point the number of people using IMAP and never deleting their mail messages from the server, will limit how much mail the ISP can store. For this reason, they would prefer that you use POP email.
Does it matter today?
Another major question is, "does it matter?" More and more people are moving to web based email services like GMail and Yahoo! mail. They are no longer checking their email by using a mail client.
By using a web interface, even if your mail provider is using POP email, you are using it like IMAP email. This is because, while you may be looking at the mail from various computers, you're still always logged on to that email server's web interface. The issue of whether POP or IMAP is better is never an issue if you never download the messages to local computer. However, if you choose to use the more powerful tools offered by an email client like Outlook or Thunderbird, choosing IMAP over POP is what you should choose when possible.
All modern email clients can handle mail in either of the two email protocols. The limitation is in the way your email provider has configured their server. If the provider's email servers are configured to POP email only, then there is no way you can use the features of IMAP.