What is the difference between HDTV and current digital TV services?

A high-definition signal enhances both the picture and the sound so that it is more crisp and more detailed than ever before with about five times the resolution. HDTV is also shown in a 16:9 ratio as apposed to the 4:3 ratio of standard TVs. Some programs are also broadcasted with Dolby Digital 5.1 Surround Sound as opposed to the stereo audio that comes with regular digital.