What is actually meant by 50 Ohm system?

Hi,
I want to understand the nature and concept of 50 Ohm system. Please view the image below to have an idea of the context of this question.

http://oi45.tinypic.com/f5877l.jpg

hi there

you need to repost that pic its a bit too small and badly out of focus to read what it is saying

Dave

Hi,

Thanks for pointing out the problem. I think it is now much better

I also have another question which image is in the following

A 50 ohm system means that the ratio V/I = 50. This is an impedance that is used for coaxial cables (for domestic use or video, 75 ohms is used). So in an amplifier the transistor might have an output impedance of say 100 ohms. A matching network would then be needed to reduce the impedance to 50. As every one uses 50, you can buy 50 ohm system components from any manufacturer.
Frank