POP and IMAP are the protocols used to read emails from remote servers.
POP stands for Post Office Protocol. The last version of POP is POP3 and was established in 1988. The last revision of the protocol was set up with RFC 1939 (https://tools.ietf.org/html/rfc1939).
IMAP stands for Internet Message Access Protocol. The last revision of IMAP is IMAP4 and was established in 1988. The last revision of the protocol was set up with RFC 3501 (https://tools.ietf.org/html/rfc3501).
IMAP was developed later than POP and it supports a wide range of operations.
Email clients (Thunderbird, Outlook, …) and web clients (Roundcube, Squirrel, …) implement these protocols to allow management of your mailboxes.
The use of these clients create confusion between what protocols allow and what the clients do.
Is a common cliché that with POP messages are downloaded and by using IMAP messages are not downloaded.
This is not completely true: for example both Thunderbird and Outlook download messages when configured with POP or IMAP.
With POP protocol, clients create in local a copy of the inbound messages from the server. Note that POP allows to read from only one remote directory.
Since IMAP has many more commands to manipulate mailboxes (write email, create directory on the server, …) implementation of this protocol gives to the user the ability to replicate in local the same structure folders as in the remote server (this is called “synchronization”). Note that IMAP allows to read multiple remote directories.
So both POP and IMAP download messages from remote. It is the client implementation that give us the perception to see messages downloaded instead of synchronized.