Ok. First, a FTP Client is a program that is used to connect to an FTP server, just as a side note.
A "null shell" means that the user doesn't have an actual shell so they can't really log into the machine and execute commands. On RedHat there is a "null" or "fake" shell already listed in /etc/shells it is /sbin/nologin.
Again, I think you will find usefull information on the linux documentation project.
http://www.tldp.org/LDP/solrhe/Secur...p29sec294.html