Pilotaware can detect other aircraft with Pilotaware boxes, and also aircraft with ADS-B.
But it can't display FLARM (used by gliders), Mode C or standard Mode S transponders (using by most light aircraft and microlights).
ADS-B is largely limited to commercial aircraft/airliners although a small number of GA have it.
Some of the latest Mode S transponders are capable of ADS-B but not all - they need an expensive approval and must be connected to a certified GPS - so relatively few people have invested in this yet. Some larger airliners have Extended Mode S transponders which send out position information which the simpler GA versions don't.
ADS-B spits out the position information frequently (as does Pilotaware), so does not depend on being within radar coverage. Mode C and Mode S transponders only transmit in respond when scanned by radar. This means at low altitude/remote areas etc, ADS-B/Pilotaware will continue to work, even when outside radar coverage.
Aircraft detected by Pilotaware can be displayed on SkyDemon in real-time; you can filter out those pesky airliners at 30,000 feet.
Range for Pilotaware is anything up to 25 km*, but Lee said he might tune that down a bit to avoid clutter.
Hope this helps
*Editted to correct 25 miles to 25km as pointed out below