HDTV signals are transmitted using either UHF or VHF frequencies. These are the radio frequencies that are received by an HDTV antenna in order for you to view free, local television over the air.
UHF (RF channels 14 - 69) signals are most common for approximately 75% of your favorite TV stations, and are a higher range than high-VHF frequencies. This means that the signal reception for a UHF TV antenna is very strong, but is somewhat limited to line-of-sight to the broadcast tower. For example, channels like ABC may be on high-VHF (RF channels 7-13) signals, so a high-VHF digital TV antenna will be necessary.
High-VHF signals are generally suited to slightly farther than line-of-sight from the broadcast towers. This means that while the range capacity of VHF reception is slightly farther, the signal is not always as strong as UHF, so a slightly larger HDTV antenna or a UHF HDTV antenna with a VHF element will be necessary.