An FTP site is based on a server program running on a so-called "host" computer. Many companies and organizations offer, for example, downloadable shareware versions of their software on public FTP servers.
If you are logged onto such a server and you cannot find the desired information, look for a file named, for example, readme, readme.txt, ndex or 00index. These files usually contain important information about the files available on the server. These FTP sites are usually public sites. You may simply log in with the user name anonymous; and enter your email address as password.
Other, password-protected, servers allow users to upload files themselves. This way websites may easily be updated and extended.