A passive device is a device that “passes” a signal through or possibly alters the impedance of the signal but “passes” it through. A general rule is if it doesn’t require power to work, then it is passive. Baluns are passive devices that change a balanced signal to an unbalanced signal (see Balanced & Unbalanced Signals for an explanation of Balanced and Unbalanced). Passive hubs typically contain only resistors between the ports. Almost all cables and cable adapters are considered “passive”, typically all they’re made of is wire and plugs. A user will typically look for a “passive” device to suit their needs, because passive devices are cheaper. But in some cases, we may have no choice but to go “active” to suit their needs.
An active device translates or strengthens the signal. As a rule, these need power to work. The Parallel Extender may appear to be passive because it runs without a power supply. But it is really an active device, pulling the power it needs from the handshake signals on the printer. In some cases, where the printer can’t give enough power to it, the Parallel Extender will need an external power supply. Converting the PS/2 style keyboard to USB requires an active device, the USB to keyboard (and mouse) connection. Connecting an old printer with Serial or Parallel port to a USB port requires an active device, ADP3141 is USB to serial and ADP3140A is USB to parallel. USB devices get their power from the USB port.
With SCSI Terminators, a passive terminator contains only resistors. The passive terminator works with low to medium speed SCSI signals, but there are some SCSI circuits that the Passive terminator will not work on. An Active terminator contains a regulator for better precision, which is needed for higher speed SCSI signals. The LVD terminator is an Active terminator made for the LVD interface.