A passive hub is basically a common connection point for cables. It provides no signal regeneration or signal processing. Passive hubs are limited because they contribute to overall signal loss, and therefore shorten the total electrical length of a physical network. You can make your own just by soldering a bunch of connectors together. Active hubs are now very inexpensive, and unless you have a specific reason to use a passive hub (low-voltage, portability, etc) you should stick with the active version.