Post Office Protocol (POP) delivers you mail from the server in a single instance. a Copy of your emails are downloaded and stored on a local destination. - In other words, POP3 does not coordinate with the server. Messages marked as read/deleted/replied to in the mail application will not show up as such on the server. This means that future mail downloads with POP3 will show all messages as unread.
Internet Message Access Protocol (IMAP) is a two-way management system for email. Therefor any changes made to emails (read, unread, deleted etc) is synchronized with the server and reflected on all your devices that has the account set up as IMAP. - Therefore, IMAP email access coordinates between the server and your mail application. Messages that have been read/deleted/replied to will show up as such, both on the server and in the mail application.