IMAP, the Internet Message Access Protocol, synchronizes email messages across multiple devices while keeping them on the server.
An IMAP client can display folders, read or flag messages, and synchronize those changes with the mail server. This makes it suitable for people who use the same mailbox on a phone, tablet, and computer.
POP3 takes a different traditional approach: it commonly downloads messages to one client and may remove them from the server, although modern settings can preserve copies. SMTP is different again; it is used primarily to send and relay email rather than to manage a mailbox.
IMAP was designed for remote access to stored mail. Secure deployments commonly use IMAPS or IMAP protected by TLS, while ordinary IMAP historically used TCP port 143.