IMAP is now the most common and widely used way for devices to retrieve email from the Mail Server. Instead of simply loading your email as independent copy onto your phone or computer, IMAP keeps the email on your server and your devices in sync. If you delete an email on your computer or phone it will also be deleted from your Mail Server.
IMAP has replaced the out-dated POP protocol, which simply downloaded email to your device but did not sync it, which meant you always had to delete the email separately from your Mail Server. See also Mail Server.