4 links
FTP Protocol Overview
freesoft.org
Very brief (1 page) overview of FTP. Part of "Connected: An Internet Encyclopedia"
FTP: File Transfer Protocol
cr.yp.to
Reference manual aiming to describe how FTP actually works (and doesn't work) in practice.
FTPLab.Com
ftplab.com
How to choose an FTP program. About tasks which can be performed by a special program called FTP Clients.