By unanimous agreement, high definition television is clearer, sharper, and visually more pleasing than standard definition television.
Why? for the same reason that a 8 megapixel digital photo is sharper than a 5 megapixel photo: there are more pixels packed together to show the same thing.
In the US, locally broadcast HD programs come in either 720p or 1080i resolution or definition. HD programs are broadcast with separate digital broadcast equipment from SD programs and from separate digital TV station towers in your area.
The further away you live from the towers, the weaker the HD signal is to your house. So, your antenna equiptment is important. Sometimes, your old roof antenna that you used for standard TV is good enough for picking up HD signals.
Almost none of the HD TV tuners for PC's are capable of picking up HD from cable and satellite set-top-boxes because the cable and satellite signals are scambled and encrypted. So, if you want to watch HD from cable or satellite, you will need to get their equipment.
Lastly, a HD tuner can not upconvert a standard definition TV program to high definition quality. However, if a local TV station sends a standard definition TV program over its digital HD equiptment, your HD tuner will deliver it to your HD television in better quality than the same program will show on a standard definition television. In other words, standard definition TV looks better on high definition euipment, generally.