FTP (File Transfer Protocol) is the standard protocol for exchanging files and/or folders over the world wide web (internet). FTP works on port 21, which is different from the HTTP port 80 on which web servers work and serve websites. To use FTP and transfer files and/or folder over the internet you will need an FTP client and FTP server. Good client is FileZilla. Servers can be configured to allow anonymous transfer of files, or can be setup to require authentication of users and apply permissions to specific users or files/folders. On Bluehost FTP accounts can be configured via cPanel. cPanel allows you to even setup different permissions for directories and setup quota (how much megabytes can be stored or transferred and permissions) per user. The tool is really handy when you have more domains and you want others to upload and setup websites. When you add addon domain via cpanel there is also an option to setup FTP account for that domain, and the account will only allow operations for files of that domain only. I asked on bluehost live chat if they allow multiple FTP connections and the answer is yes. This can speed up process when downloading/uploading files. Really handy!