The FTP (File Transfer Protocol) Server app uses ProFTP (http://www.proftpd.org/) - a highly configurable, Open Source, FTP server software application. FTP can be used as a standalone service to provide file access (upload or download) by authorised users/services or in parallel with Flexshare - ClearOS's flexible document sharing application.
To learn more about this app, click here
Documentation for ClearOS 6 can be found here.
Documentation for ClearOS 7 can be found here.