Google had revenues of $37.9 billion in 2011. Of that total, almost all (96%) came from advertising revenues.
The following infographic from WordStream (thanks to Wordstream for preparing this!) breaks that number down into the top 10 industries that provided Google with 60% of that ad revenue. It further identifies the top five brand names in each of the 10 categories and gives examples of popular keyword searches and the cost per click for those words/phrases.
Take a look for some very intriguing hard data on actual spending on online advertising and scope out where the industry your business is involved in sits in the hierarchy.
Click the infographic to enlarge.