POP3 and IMAP are two separate protocols to access e-mail stored on the server.
POP3 is designed for devices that have limited/sporadic Internet connectivity and is optimized for limited server-side storage.
(Downloading all of the messages and storing them locally).
IMAP keeps e-mail on the server and synchronizes Messages and Status required (read, replied to,...).
All of the Users E-Mail Clients access E-Mail Messages from the server as required and may cache them locally.