SMTP (Simple Mail Transfer Protocol) allows two mail servers to communicate using a simple language, and provides a step-by-step protocol for exchanging information.

SMTP delivers mail to a central location, where the user can either log in and read it directly or use a client/server mail protocol to read it remotely.

IMAP (Internet Mail Access Protocol) is client/server mail protocol.

IMAP is designed to keep mail on a remote server and let the user interact with it there.

People with fast connections and multiple machines usually use IMAP so they can read mail from whichever machine they happen to be on without losing access to the mail they read elsewhere.

POP3 (Post Office Protocol Version 3) is client/server mail protocol.

It is designed to forward a user's mail to a single machine, where the user can go offline and read it, if necessary.

In general, people with slow connections (dial-up or otherwise) tend to use POP3 because they can connect and download their mail without having to keep the connection open afterwards.