Three companies are at the forefront of modern personal computing: Microsoft, Apple, and Google.

After reading through the most recent SEC-mandated financial reports for each company, I was inspired to put together these three pie charts. The data paints a vivid picture of where each company's revenue comes from.

Microsoft is a software company. Apple's a hardware company. But what business is Google in?

Microsoft

A well-rounded software company that has been surprisingly successful in recent years with its consumer entertainment devices, the Xbox 360 and Kinect. The company has high hopes for its online services. (Source: 10Q FY12 Q2 , filed January 2012)

Apple

The most successful reinvention of a hardware company ever. The iPhone and iPad didn't exist five years ago, and last quarter they accounted for more than $33 billion in sales. That is mind-boggling. (Source: Q1 FY12 Form 10-Q [PDF], filed January 2012)

Google

"We generate our revenues almost entirely from advertising... Advertising revenues made up 97% of our revenues in 2009 and 96% of our revenues in 2010 and 2011." (Source: Form 10-K for the fiscal year ended December 31, 2011 , filed January 2012)

If you want to understand why a company acts the way it does, just follow the money.