FTP (File Transfer Protocol) is a standard protocol for transferring data(files or folders) over the internet. FTP also supports other commands too including changing permissions, creating folders, deleting files, moving files and other related tasks. It requires client and server to work. Most popular client is FileZilla, free software, and is available on linux, windows and mac computers. FTP is good for transferring large number of files/directories, for small changes you should use the File Manager cPanel tool.
Justhost FTP
Justhost offers FTP to its customers. You can have unlimited accounts, and each account can have specific permissions and quota (transfer of megabytes allowed). Accounts can be created in cPanel control panel, where you can also edit them. This is good if you want to give your friends or clients accounts with specific settings and access to some parts of your website.