IMAP & POP3

IMAP and POP3 are email protocols (roughly, a protocol is a set of communication rules).

IMAP allows a user to store email messages on the server until deleted, meaning that the user can access an email account from many different computers. It also means that the user's email client [?] will synchronise with the server, so that the management of messages on the client will be reflected on the server.

POP3 protocol requires that the user must either manage messages on the server through webmail, or download the messages to the email client, which may mean the message is deleted from the server. When compared to IMAP, POP3 has many disadvantages, but not all mail providers supported IMAP.

IMAP stands for Internet Mail Access Protocol.
POP3 stands for Post Office Protocol version 3.