With the digital TV transition last year, most of the broadcasters switched over to a vastly improved method of transmitting TV over the air (OTA). Those of you who grew up with OTA TV remember that your analog TV got two types of channels: Very High Frequency and Ultra High Frequency. VHF channels were numbered from 2 to 13 while UHF channels were larger (e.g. 36, 44, 54, etc.). The channels were quite simply numbered in order of increasing broadcast frequency.
Digital TV uses the same two VHF and UHF frequency bands, but introduced some differences.
With Digital TV, the channel numbers are now "virtual" meaning that the number no longer indicates the frequency. Channel 2 in your area could be anywhere on either the UHF or VHF band. Normally, you don't need to care, but since UHF and VHF require different antennas it becomes important when setting up your antenna.
For example, when I first got my digital TV antenna I wondered why I didn't receive channels 7 and 11. When I looked up the channels I noticed that they were broadcasting from the same towers that was broadcasting others channels that I received perfectly. Why did one channel come in perfectly while the other did not? Later I discovered that 7 and 11 were on the VHF band.
Digital TV is mostly on UHF (for now)
When Digital TV first began in the US (1998), broadcasters were allocated channels in the UHF range. This was because it was the least crowded and didn't overlap with existing analog channels during the transition.
Now that Digital TV is mandatory and most of the analog stations have shutdown, you may see broadcasters move back to the VHF band (my guess). However, they will probably only use part of VHF.
VHF is divided into two halves. If were are referring to the old analog channel numbers, the two halves are channels 2-5 (VHF-low) and channels 7-13 (VHF-high). For some reason, Digital TV doesn't seem to like the VHF-low band and supposedly causes undesirable interference. For this reason almost all of the Digital TV stations use the VHF-high region.
When you look at those large analog TV antennas that remind you of a "fish-bone", the largest bars are for the lowest frequencies. One benefit of this is that if your Digital TV VHF antenna can be smaller.
Sub Channel numbers
Another difference you will notice with digital TV is that the channels have dots in them (e.g. 2.1, 4.2, etc). The first number is the virtual channel while the rightmost number is called the subchannel. With digital TV broadcasters can use their assigned channels to send out multiple video and audio streams. Each sub-channel is a full channel and has different content. Some stations use sub channels for different languages (e.g. content in Spanish, Mandarin, etc.). In any case, it means more channels for you!
From good to awesome picture quality
Broadcasters also have control over the quality of the pictures that they transmit. Some content may be transmitted is full High Definition (1080i), while some may be in lower quality. One big advantage of broadcast TV is that broadcasters actually have more bandwidth available per-channel than do satellite and cable operators. Trying to cram 100s if channels onto a cable has it's limits! This is why high definition sports looks fantastic on broadcast TV while satellite/cable looks only so-so. You especially notice this when there is particularly fast action, fast panning, and lots of dynamic range in the picture.
Lots more channels with Digital!
If you live in a large city, you will be pleasantly surprised at how many channels (and sub-channels) that are available. I currently get over 65! and the list seems to increase as new broadcasters come on line. Now, I have to say that, just like with cable, I wouldn't watch all of these. Some of the stations broadcast in foreign languages (of no interest to me, but not to a large percentage of others). Other content may not be interesting for other reasons. However, I get all of the major networks (in fantastic quality), several PBS channels, and lots of local content. Combine this with a high quality DVR to record what I want / when I want and you have no shortage of interesting things to watch for FREE!