If you've seen digital TV sets, you undoubtedly noticed their distinct shape compared to regular TV sets. DTV (remember, HDTV is just part of this) uses a 16:9 aspect ratio, about the same aspect ratio as a movie screen. And if we had to pick a movie, "Back to the Future" would be appropriate.
In the first half of the 20th century, movies had a 4:3 aspect ratio. Frames of film were 1" wide and 3/4" high. When TV industry leaders got together in 1941 to set standards for their new technology, they decided to adopt the same format as the movie industry. It was a good idea at the time.
But then the movie industry changed. You've probably seen those old movie trailers boasting "Cinescope" or "VistaVision." In the 1950's, movies went to a widescreen format, promising theater patrons a more exciting experience by filling more of their peripheral vision.
Today when you watch movies on TV they are "formatted to fit your screen," or they might be letterboxed (see image above), using black bars taking up the top and bottom of the screen so the image you see has a 16:9 aspect ratio.
So really, by adopting a 16:9 ratio as the standard for digital TV, the TV industry is just getting back in step with the movie industry.