June 12th, 2009 was the date mandated by the US Government for all full-power television stations in the US to cease transmitting their analog signals and only transmit a digital signal.
Older televisions do not have tuners capable of receiving the digital transmissions and a Digital to Analog tuner will be needed for those older TVs to continue to be usable for broadcast TV viewing.
In many cases, the TV antenna that you already use can be used to pick up the Digital broadcast signals.