FTP is an acronym for File Transfer Protocol. SFTP is an acronym for Secure File Transfer Protocol. The protocol is used to conduct a file transfer over the Internet. Typically that transfer is done between a client machine, such as you and your computer, to a server.
An S/FTP client is software that is installed on your computer that helps you perform an FTP transfer. There are many different S/FTP programs that can be used. The OSU Computer Helpdesk provides user support for WinSCP (for Windows) and CyberDuck (for Mac) and has some great on-line instructions in their Helpdocs website for how to use the software.
Please note that when uploading files onto the CWS servers, you must use the SFTP (Secure File Transfer Protocol) method as described in the CWS Hosting FAQ.