What is FTP?
File Transfer Protocol (FTP), is a very simple and easy way to transfer files from your computer to a server. I'm actually oversimplifying it, but it's basically the old-school way to upload files to your website. Some developers still use it to modify code on their WordPress files.
All you need is a FTP client (Filezilla seems to be the standard nowadays), a login and a password, and you are ready to upload and download files.
SFTP is the more secure cousin of FTP, and that's the one we work with on our servers.