File Transfer Protocol, it's pretty standard, if you need to connect to a server you can use a gui like Filezilla ( http://filezilla-project.org/ ) which makes very easy to connect, see the files available and upload/download. When transferring files over the net, ftp is just an easy way to move the files back and forth. It's been around almost forever. (as far as the net's concerned)
other options are openSSH, which has an "sftp" command, and I think it's much better, but if you're working with someone else's server, then you don't have the choice.
If you're trying to get files to friends, you can host an ftp server (even though openSSH-server is easier IMO) and then you give them the connection info, and they can log in and upload/download files. It's easy once you learn the commands (of course filezilla automates most of it)
Last time I used ftp, was uploading files to my webhost, when you sign up they give you your account information and you upload the .html files that you want them to host via http. Also when my friends want to share pictures or something from their home computer, they just send me their IP address to their computer running an ftp server, and i can grab what they wanted me to have.