When you compare HD television to standard television you will see many differences, most of them are not noticeable from simply looking at the television set alone, but are noticeable when viewing cable programs broadcast in HD.
HDTV has twice the linear resolution when compared with standard television. This means that the viewer, when watching a show with HD, will notice many more details when compared with a viewer watching the same program with standard television.
HD television also uses more aspect ratio when compared to standard television. Aspect ratio is the displayed width divided by its height. High definition television will have an aspect ratio of 16:9 while standard television is 4:3. This basically means that viewing a television program with a higher aspect ratio will mean a clearer, more precise view of what is being shown when compared with a smaller aspect ratio.