Whenever you set up a mail client program on any of your devices, you need to decide whether or not you will use Internet Message Access Protocol (IMAP) or Post Office Protocol (POP3) settings to access emails stored on the mail server. These settings only apply to mail clients, as opposed to webmail, and determine how your emails are stored and how the various devices you use to view mail are synchronized with the mail server. Carefully choosing which setting you use is important when setting up new mail clients, and for guaranteeing that you have maximum control over your inbox.
The IMAP setting can be considered the default setting for the average email user. When mail clients use IMAP, they rely on the mail server to provide them with all of your mailbox information. This also keeps all of your devices perfectly synchronized with each other and the server; for example when you delete an email on one IMAP device that email disappears from all of your other devices as well. It makes managing crowded emails a much more efficient process.
POP3 effectively works the opposite way. Any device that uses POP3 for their mail client will download copies of all of your mailbox items (inbox emails, sent emails, etc.) and store them independently from the mail server. If you delete an email from a POP3 connected device, that email will still be present on all of your other devices. Emails are also backed up on individual devices, instead of being stored permanently on the mail server.
Because POP3 connections require consistent management for each device that uses it, it is normally recommended only for email accounts that are used for numerous different tasks, such as a combined business and personal email where different devices are used to check different content in the same inbox. IMAP settings make it easy for a single email user to manage their mailbox from multiple devices and are best utilized for personal email accounts and specialized business accounts. Switching between the two is fairly easy and your email provider always has the settings you need to change between POP3 and IMAP.
NOTE: POP3 refers to the 3rd (and current) version of the Post Office Protocol. POP and POP3 are synonymous.