You've probably noticed that there are many more user accounts defined in /etc/passwd than actually log in to your system. This is because various Linux components use user accounts to enhance security. Typically, these system accounts have a user id ("uid") of under 100, and many of them will have something like /bin/false listed as a default shell. Since the /bin/false program does nothing but exit with an error code, this effectively prevents these accounts from being used as login accounts -- they are for internal use only.