What is the difference between Standard Definition (SD) and High Definition (HD)?

Our Low Cost TV Ads are produced in HD (high definition). 

These pictures illustrate the basic aspect ratio difference between SD and HD:

SD screen
4:3 aspect ratio (almost square) or 16:9 letterboxed (cropped with black bars). The format of older television sets.

HD (wider) screen
16:9 aspect ratio. The format of modern day rectangular screens.