From an old Economist article : 

Mr Buffett has consistently beaten the market by buying good-quality firms that he is confident he understands, typically outfits operating in a relatively stable industry. His preferred acquisitions have a hard-to-replicate advantage over their competitors—a popular brand, say, or a degree of monopoly power—that he likes to describe as a protective “moat”. He also favours firms with a strong ethical culture, and management that is interested in doing a good job, not just making money. If he gets the shares when they are cheap (just after Coca-Cola’s “new Coke” debacle, for example), all the better: but “it is far better to buy a wonderful company at a fair price than a fair company at a wonderful price,” he says.