Synchronize! works with folders on disks which are on the desktop. A "direct connection" means that both of the disk icons appear on the desktop.

A direct connection can be made via a SCSI connection with some PowerBooks or with File Sharing and an AppleTalk connection (either locally or with AppleTalk Remote Access and a modem).

Once a direct connection is established, create a Synchronize! settings document which identifies the folders on the two disks which are to be kept identical, and you're ready to synchronize those folders!