POP and IMAP are both common protocols used by mail clients to connect and retrieve mail from your server. Depending on the behavior you want from your mail client and the server one option may work better for you than the other, so knowing the differences will help you choose the one that works best for you.
This account type connect to the mail server and retrieves any new messages, creating a copy of that mail in the email client. Any changes, such as reading, sorting, or deleting of messages, are only reflected locally and the server is unaffected.
POP has the advantage that you do not need to store mail on your server for extended periods of time. Once a message has been retrieved with POP it will exist in the local mail client (such as Outlook) unless manually deleted. Mail clients can also remove retrived messages from the server after a specified period of time to automatically help you manage storage space.
Multiple devices configured with POP means you will be sorting, deleting, or making email as read multiple times since each client has an independent copy. For mailboxes that are used by multiple individuals this is a great feature. Please be aware, however, that if POP is configured to remove messages from the server immediately it can adversely affect other users on a shared mailbox.
An IMAP account type not only retrieves new mail from the server, but also syncs back any changes so the email client and server data are always consistent. This allows you to read an email or sort it to a folder once and have the changes visible on all of your devices.
IMAP accounts are great for syncing data between multiple devices and for making sure that your mailbox looks the same when you log in through the web interface. If you check your email from a mobile device and a desktop this option may save you time in managing your email data.
One downside that can come up is storage space when you receive a lot of email, or email with large attachments. Because data is stored on the server indefinitely, unlike POP, your usage only goes down when data is deleted. Changes made on the server, such as removing email, will be reflected in your local client as well. Keep this in mind when configuring multiple clients with both POP with IMAP for the same email account, if you have POP set to remove messages from the server after a set time they will disappear in your IMAP account as well.