POP3 stands for "Post Office Protocol Version 3" and is a protocol for receiving e-mail. This protocol enables offline mode. E-mails are transmitted from a server to the e-mail program you use and then deleted from the server, if not set otherwise. You then only have access to already retrieved and sent e-mail through this e-mail program. You can read e-mails which have already been downloaded without an Internet connection.
SMTP stands for "Simple Mail Transfer Protocol" and is a protocol for sending e-mails. SMTP transmits your e-mails from your e-mail program to a server and then forwards them to the respective recipient.