There's a lot of (quite technical) information here:
http://bitbucket.co.uk/work/aspect_ratios.html
But at least the pictures give you some idea of how the images can be displayed when the TV is set up correctly.
Widescreen has been a great marketing gimick that allowed the industry to sell lots more TVs. Unfortunately, no one seems to have settled the argument as to what happens when the aspect-ratio (shape) of the content and the TV don't match. This has lead to much confusion and many different implementations.
Personally, I can't see what's wrong with showing the picture in letterbox (black bars at the top/bottom) or pillarbox (black bars at the side) format if required. But then I seem to be in the minority. I get irritated watching in S-T-R-E-T-C-H-O-V-I-S-I-O-N and possitively fuming when there are black bars at the top and bottom as well.
Further explanation:
http://www.htexplained.com/abridged/Chap%202.htm