The widescreen picture is 16 units wide and 9 units high. And as Tapirman says, the ratio is called 16:9.
If you wanted to watch the older "aspect ratio" of 4:3 (4 units by 3 units) on a widescreen telly, the top and bottom of the image is chopped off and the image.enlarged.
Most archive programmes and those from foreign broadcasters are in the 4:3 format.
For a channel like BBC1 broadcast in widescreen on the digital platforms, the loss of picture information and degredation of quality in the process of coverting between 4:3 and 16:9 is deemed too high. So these programmes are broadcast as a 4:3 box within a 16:9 picture. Hence you get, thick black lines at either side of the picture. Or in Sports coverage annoying blue lines!
For a thorough guide try clicking
http://www.bbc.co.uk/guidelines/delivering_quality
/delivery_widescreen.shtml