An FTP client is a software application used to send and receive files from an FTP server in Internet.
Nowadays many people use FTP clients to update the files used in web servers but the main purpose is the storage of files of any kind organized into directories.
The FTP servers may allow public access to their files or allow access only to certain users. These private FTP servers use usernames and passwords to restrict the access to their files while the public FTP servers allow anyone to access their files using anonymous as username and any email address as password.
Public FTP servers are also called anonymous FTP servers and they usually store their public files inside a directory called pub.
It's not unusual to find FTP servers that keep a complete copy of other servers inside a directory called mirror.