FTP is standard Internet protocol used for transfer computer files between server and client. It uses separate control and data connections between the client and the server. FTP users need to authenticate with server through user name and password. It is simplest and most secure way to exchange file over the network.

Login:

Simply through normal user name and password we can login to FTP. The user name is sent to the server using the USER command, and the password is sent using the PASS command.  This sequence is unencrypted “on the wire”, so may be vulnerable to a network sniffing attack. If the information provided by the client is accepted by the server, the server will send a greeting to the client and the session will commence. If the server supports it, users may log in without providing login credentials, but the same server may authorize only limited access for such sessions.

Anonymous FTP:

A host that provides an FTP service may provide anonymous FTP access. Users typically log into the service with an ‘anonymous’ (lower-case and case-sensitive in some FTP servers) account when prompted for user name. Although users are commonly asked to send their email address instead of a password, no verification is actually performed on the supplied data. Many FTP hosts whose purpose is to provide software updates will allow anonymous login.

Source: Wikipedia