Port 21 is the default File Transfer Protocol (FTP) control port. Servers use port 21 to initiate the connection and receive the username and password from the FTP client. After these credentials are established, the server initiates a connection on another port for data transfer. The default data port is port 20.
FTP is one of the earliest network protocols still in use. It dates to 1971, and is older than the TCP/IP protocol it uses for connections. Different types of FTP offer varying levels of anonymity and security; this is necessary because FTP is not designed to be a secure or anonymous protocol on its own.