If you've ever set up your email in an email programme (Outlook, Windows Live Mail, Apple Mail etc), you will have been asked if you want to set up your email using IMAP or POP3. While you will generally find that either of these work (certainly the email services we provide will work with either of these), you may not be aware of the difference between these.
POP3 (Post Office Protocol 3) is the older of the two methods of connecting. When you set up your email to use this, your email programme has a look for new email on your email server. If it finds some, it will download it to whatever device you're using, and then delete the email from the server. Now, the only place that email exists is on the device you've just downloaded it to. You can equate this to exactly how the post works. Once the postman has delivered the letter to you, you now have the only copy of that letter. On modern email clients, it is possible to delve into the settings to stop it deleting the email on the server. We wouldn't recommend this though. If this is how you want things to work, then IMAP is the method you want to pick.
IMAP (Internet Message Access Protocol) does things differently. When you set up your email to use this, your email programme gives you a view of all the messages on the server. In this case, the emails always exist on the server, and your email programme is then used to manipulate the emails directly on the server. This means that any number of devices can connect to your email account (e.g. a PC, a phone and a tablet), and they will all see exactly the same list of email. If you delete an email via any of these devices, this will delete the email at the source, so it will be deleted on all devices. Labouring the analogy somewhat, this would be like if all your letters were always stored at the Post Office, and whenever you wanted to see one of them, the postman runs to your door and lets you see a copy, but the original stays at the Post Office. If you tell the postman you don't want that letter anymore, they go back and get rid of the letter at the Post Office.
In almost all cases, we'd recommend using IMAP. However, in cases where you have a high volume of email that you always want to access from one device, POP3 may be the answer. Because POP3 deletes all email from the server as it is collected, this means that your email account uses minimal space on the server, which can help in cases in which you need to cut down the amount of server space you are using to keep costs down. Once downloaded, the only limit is the hard drive space on the device you are using, which is usually far greater than you will get in a hosting package. If you use this approach, just be careful that you take adaquate backups. Otherwise, if the hard drive with your email on dies, you will lose it all forever!