Internet Message Access Protocol
With IMAP, all your mail is stored on the server and you use an email application on each of your devices to read it. It is ideal for when you regularly view your email from more than one place. E.g. from your desktop computer at work, your laptop at home and your mobile... everywhere.
To use IMAP effectively, you need to make sure that each mail application on each device is set up to use IMAP.
Your mailbox has a quota limit. If you run out of space you will not be able to download new emails so it's important to keep an eye on this. You can check your remaining quota in the management console.
Post Office Protocol
POP (or POP3) is an older method of retreiving mail and is suitable if you have no need to check your email in multiple locations.
When you check your email, it is downloaded and stored on your computer and then deleted from the server, so it is important to have your own backup in place.
Some mail applications allow you to keep mail on the server for a certain number of days before deleting them, meaning you can check recent messages on other devices over IMAP or via Webmail if necessary.