FTP is an acronym for File Transfer Protocol.
It is a standard network protocol used to transfer computer files between a client and server on a computer network.
When uploading website files to your server, you need an FTP client, which is the program that helps you move the files.
Open Source (free!) FTP Clients
FileZilla works on Windows, Linux and Mac. It offers a full range of features and is fairly easy to use.
Fetch is a reliable, full-featured file transfer client for the Apple Macintosh whose user interface emphasizes simplicity and ease of use. Fetch supports FTP and SFTP, the most popular file transfer protocols on the Internet.
Cyberduck is an easy to use interface for Windows and Mac that helps you connect to FTP (File Transfer Protocol), SFTP (SSH Secure File Transfer), WebDAV (Web-based Distributed Authoring and Versioning), Amazon S3, Google Cloud Storage and Rackspace Cloud Files.