POP3 (Post Office Protocol)
Here the mail person accesses the mail box from say a PC and the mail gets accumulated on a server. So in POP3 the mail is downloaded to the PC at a time interval which can be specified by the user. POP3 is used when the mail is always read from the same machine, so it helps to download the mail to it in advance.
IMAP(Intermideate Mail Access Protocol)
Here the user may access the mail box on the server from different machines so there is no point in downloading the mail beforehand. Instead when the mail has to be read one has to log on to the server. (IMAP thus provides authentication) The mailbox on the server can be looked upon as a relational database.