As of June 2009, television broadcasters are required to broadcast a digital signal, or DTV. However, this does not necessarily mean there is a requirement that all broadcasters broadcast in HDTV, or high definition, "hi def," TV. Most do, however, and many customers appreciate the increased clarity that HDTV provides.
Traditionally, television signals were transmitted in an analog method; however, digital provides a more modern and technologically advanced manner of transmitting a signal. More information can be transferred digitally than can be transferred using an analog connection, and the information can be transfered faster, more efficiently, and in a higher quality. For this reason, as well as other reasons including the opportunity to open up more analog broadcast channels, the government mandated that all television service providers switch to digital television.
For anyone with a newer television and/or cable television, this switch should have been virtually seamless. New televisions are equipped to decode digital signals, as are cable box decoders. However, for those with older televisions that are not yet digital ready, the television signal from the new digital television will not work. As such, converter boxes were made available, often with a subsidized coupon from the government, allowing those who are unable to receive digital signals on their existing TVs to continue to get programming. The converter boxes simply translate the signal from digital to the old analog connection.
It's important to note, however, that just because you are receiving digital signals does not mean they are high definition or HDTV. High definition requires more pixels per inch, and there are generally different filming techniques and other special requirements. Broadcasters and television stations do not have to use or offer these high definition features and not all channels are available in HDTV as of January 2011.