In traditional analog television there was a 1-to-1 correspondence between the channel number which the station advertised and the RF channel frequency it occupied. For example, an analog station advertising itself as "Channel 11" occupied the frequency allocated to channel 11 by the FCC. The frequency occupied by channel 11 covers 198 to 204 MHz and is centered at the mid-point between those frequencies (at 201 MHz). This meant that if you knew the channel number of a TV station you could force your analog TV to that station frequency by entering the channel number, no scanning necessary, and the TV would go to the correct frequency.
In DTV, however, stations broadcast information such as their call letters and their "virtual" channel number continuously as background data, and the RF channel they use might be different from the virtual channel number. A channel might advertise itself as "DTV 68" or "68-1" but actually use RF channel 42. Your DTV converter box (or digital TV) can't know this without scanning all of the channel frequencies to find it. When doing a scan, your DTV tuner is checking each of the possible RF channel frequencies for a signal, and if one is found, memorizing the advertised channel number.
The advantage to this is that stations can continue to advertise themselves as the same "virtual" channel number as their previous analog channel number, yet in reality transmit their RF signal on a TV channel number which is different from the "virtual" number.
After the transition some DTV stations began to transmit their digital signals on the same RF TV channel as their advertised virtual channel number (DTV virtual channel 11-1, in the example above, moved to the RF TV channel 11 on June 12, 2009, replacing its original analog transmission on channel 11 with a digital transmission on channel 11). Others did not (DTV virtual channel 68-1 transmits on RF TV channel 42).
A disadvantage to virtual channels is that you must be able to receive all the channels in your area in one scan. If your antenna has to be oriented in one position or direction to receive some channels and in another direction to receive others, it may be almost impossible to pick up and memorize all the channels in a single scan. In that case you must rely on the ability of your converter box or DTV set to either perform a "channel add" scan or manually tune to the RF channel of a station (not all have this capability). For the latter, find the RF channel number of the station from this table under the column "Channel Numbers/Digital" enter this number into the RF channel box in the "manual channel add" or "manual tuning" function of your converter box, then adjust your antenna until the signal can be received and added to the memorized channel list.
Due to higher bandwidth efficiency, DTV broadcasts can fit more than one channel of video/audio information into the same 6 MHz wide RF channel, but the additional channels are usually at lower video resolution than the primary or contain images which do not change quickly (such as weather maps). For example, if the primary channel is virtual "5-1", there may be a virtual "5-2" and "5-3" as well, all occupying the same RF channel (which could be any RF channel, not necessarily RF channel 5). Usually the suffix "-1" (such as "5-1") indicates the primary channel and has the highest resolution. The higher numbered virtual channels ("5-2", "5-3" in this example) usually must have lower resolution or be slowly-changing images in order to fit within the boundaries of one RF channel.