ftp means File Transfer Protocol and is a means of transferring files from one computer to another by using the network. It comes into its own for transferring large numbers of files, or large single files (more than 1MB) and can be used across the entire Internet. Note, however, that if you wish to send a single file of modest size to a particular person, it's best to use an email attachment.
To use ftp you need
A login name and password for the remote system unless it is a public source and allows the use of anonymous ftp.
An ftp client, which is the program used to transfer the files. The ftp command is available on all ISS Unix services; in addition AxY FTP, available only on Sun workstations, provides an easy to use graphical interface for file transfer.
Anonymous means that you do not need a special login name assigned at the remote site in order to access the files being made available. Instead, you use the login name anonymous, and use your e-mail address as the password.
There are thousands of anonymous ftp sites on the Internet. Some of them are central repositories for material (software, information, data, etc) relating to certain areas, for example certain types of computer, or specialist areas in research or applications.
There is no "directory enquiry service" for anonymous ftp sites. The names of most sites are passed on as part of the information being passed around a particular group of people (for example, chemists know about the best ftp sites for chemistry software, just as they know about the best e-mail lists, news groups and web sites).
In fact, many ftp sites are now accessed directly from the Web - an example is the UK Mirror service which mirrors many public archives and largely does away with the need to use ftp. You can, however, still use ftp to connect to ftp.mirror.ac.uk if you prefer: this can be faster and more efficient if you know more or less what you want to retrieve, and is certainly more efficient if you will be retrieving many files.