What is the difference between audience rating and audience share?

What is the difference between audience rating and audience share?
Audience Rating is the percentage of all possible homes with televisions that had the program on. Share is the percentage of all homes that were watching TV at the time, that had the program on.

So Share will be higher than the Audience Rating, especially for low-viewership timeslots. More specifically, the share are the people that are actually paying attention to the program. Rating is just those who have the TV turned on.