IMAP (Internet Message Access Protocol) and POP (Post Office Protocol) are protocols which will allow you to configure your email on a mail client and manage your messages from that program. This article will tell you how to decide which one you should use.
The main difference is when you take action with IMAP, that action is reflected on the server. If you delete a message, that message is also deleted from your mail hosting server. With POP messages are downloaded on your computer, and you are managing them locally. Marking a message as read or deleting a message will not affect your emails on the server. That is why you may find a lot of unopened mail when you login via webmail, even though you have already read the messages in your POP configured mail client.
You will probably prefer IMAP if you use your email on different devices. If you read a message on your laptop, it will also be marked as read on your iPhone / iPad. If you send a message from your laptop, it will also appear in the ‘Sent’ messages folder on your iPhone / iPad and other devices.
You will probably prefer POP if you A) would like to keep a local archive of your messages i.e. when changing your mail hosting provider and mail from the old server will be deleted; B) if you have a limited mailbox size and want to store your email locally or C) if you would like to make sure you have a backup copy of your emails on the server in case you delete something important
Caution! During configuration of your POP account, you need to exclusively state whether or not messages will be deleted from the server when downloaded to your device (or to keep copies for a certain time and then delete them). A number of mail clients and apps will not present you with the option to choose and will do one or the other by default. Make sure you check your settings!