http://adwords.blogspot.com/2007/07/discover-your-share-of-voice-with.html

https://adwords.google.com/support/bin/answer.py?answer=52760

   Here is the quote....

Impression Share (IS): The percentage of times your ads were shown out of the total available impressions in the market you were targeting. This metric is available at the campaign and account level for search.

Lost IS (Rank): The percentage of impressions lost due to low Ad Rank (cost-per-click bid x Quality Score).

Lost IS (Budget): The percentage of impressions lost due to budget constraints.