From The TV IV
A digital channel is the channel position on the digital spectrum on which all full-power (and some low-power) over-the-air TV stations broadcast their signals. Digital uses a series of "1" and "0" as transmission data, compressed so stations can add additional channels--or subcarriers--to their rosters. On June 12, 2009, digital became the broadcast TV standard as analog TV--which had been the standard since TV's creation--was discontinued. The analog spectrum will be auctioned off by the government for use as emergency two-way radio transmissions.
When the digital spectrum went into effect in 1996, it originally ran from channel 2 to channel 69, the same as the analog spectrum. More than 3/4ths of America's stations were broadcasting on positions that were different than their analog position. Many favored UHF positions as the frequencies traveled without the restrictions and interference issues that VHF stations would have. To keep viewers from being confused as to where their favorite local stations would be, the FCC imposed the Virtual channel.
February 17, 2009 was the original date for all full-power stations to shut off their analog and broadcast solely in digital, although some areas shut theirs off even before that date (Wilmington, N.C. and Honolulu HI among them). However, it was found that over half the viewers in the U.S. were still not prepared for the digital transition in spite of crawlers, PSAs and program-length instructionals. Congress passed a bill that President Barack Obama signed to push the analog shutoff to June 12, 2009. At that point, channels 52 to 69 on the digital spectrum were discontinued and any stations physically broadcasting in that range will have relocated to a position of their choice (determined ahead of time) below channel 52. Somewhere down the line, digital channels 2 through 6 will be discontinued, meaning the digital spectrum will run from channel 7 to channel 51.