If the way that Google performs could be replicated, then we would all be doing it! However, the way Google do things is complex and in many ways secret as this is what the company is based on. However we do know that Google are bringing in around $3 billion a month from advertising, the main earner being text adverting. Therefore, what are the top categories, well according to research carried out by Wordstream, it seems that the top three are insurance with 24%, loans with 12.8% and then mortgage with 9%, within the insurance sector the top bidders are willing to pay $54 per click. This means that for the start up or smaller business this is hard to compete with the big boys.
Therefore, it appears that the answer is that Google are making most of their advertising money from the financial sector, which is no real surprise! Wordstream have produced this interesting info graphic that shows just how the Google money making machine works.