Based on his impressive track record during the past few decades, Warren Buffett is a source of inspiration when it comes to investment ideas. Take a peek through Berkshire Hathaway's massive portfolio, and you'll find some dominant companies.

The third largest holding is American Express (NYSE: AXP). The conglomerate owns almost 21% of the shares outstanding in this top financial company. But before you rush to follow in the Oracle of Omaha's footsteps and buy the stock, here are three things you need to know.

American Express is similar to and Mastercard in that all of these companies operate payment networks that connect consumers and merchants to facilitate commerce. But Visa and Mastercard don't extend credit themselves. Instead, they rely on banking partners that take on the credit risk.

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Source Fool.com