What is the difference between IMAP and POP3?

imap or pop?
Setting up an email account on a third party client (most commonly: Microsoft Outlook for PC, Mail for Mac) seems like it should be easy. You put in your login, password, incoming and outgoing server information and viola! However there is one step that can be a bit confusing.
Do I select IMAP or POP3?
Here’s the basic rule of thumb:

