The mount helper is a program, located in /sbin. It is called mount.xxx where xxx is the name of the filesystem. If the mount command is called using mount -t xxx it searches for a executable, called /sbin/mount.xxx and runs it with the parameters, given to the mount command.
The OFS mount helper mounts the remote share and calls the ofs with the appropriate parameters.
The mounthelper is automatically called by the mount command, when the filesystem ofs is desired. As Device, you give a URL, which contains the remote filesystem as protocol and the share as path.
# mount -t ofs filesystem://path/to/share /some/local/mountpoint
In order to mount a samba filesystem through OFS, you use (only as root of course) the command:
# mount -t ofs smb://fileserver/share /some/local/mountpoint