All those blank spots in the broadband spectrum where no signals are being broadcast are known as white spaces. They're all over the place and all kinds of providers want to take advantage of them.
So seven major tech companies — Comsearch, Dell, Google, HP, Microsoft, Motorola and NeuStar — have joined together to basically catalog all these white spots. The Federal Communications Commission in November said unlicensed devices could use those white spaces; this database will be a mandatory checkpoint for those devices to ensure they don't interfere with TV signals.
The administration of the database will be "open and non-exclusive," meaning they're not going to be the only ones that can access it.
The switchover to all-digital TV, which was pushed back a few months, was the impetus for the FCC's decision and the White Spaces Database, as a lot more signals can be crammed into a digital spectrum. The only space left in analog was in the UHF band, where the stations were hard to get even with old-fashioned rabbit ears.
Basically "white space devices" are any devices that will make use of these spots on the spectrum. They can include wireless Internet, wireless microphones, medical devices and others. Apparently Broadway theater producers and sports teams are concerned that the filling in of the spectrum could affect their performances and events.