Before 2002, Google was barely bringing in any revenue. 

Then Larry Page and Sergey Brin learned of Goto.com, a search company founded by serial entrepreneur Bill Gross. 

Gross had what Page and Brin didn't: cash flow.

He "introduced a twist" to the business model, says Slate writer Will Oremus. "[P]aid search. Like the way companies bought ads in the Yellow Pages, websites could pay for top placement on the GoTo.com results page for a given keyword. This would push down spam results, Gross reasoned, because companies would have an incentive to buy ads for search terms that were actually relevant to their products." 

So the Google guys made their own version of the mechanic — called AdWords Select — and revenue took off.