"The central difference lies in ownership: whereas nobody is sure who owns public companies, startups go to great lengths to define who owns what. Early in a company’s life, the founders and first recruits own a majority stake—and they incentivise people with ownership stakes or performance-related rewards. That has always been true for startups, but today the rights and responsibilities are meticulously defined in contracts drawn up by lawyers. This aligns interests and creates a culture of hard work and camaraderie. Because they are private rather than public, they measure how they are doing using performance indicators (such as how many products they have produced) rather than elaborate accounting standards."
And a related article, Reinventing the deal, says 'America’s startups are changing what it means to own a company'.
"Working this way is not easy. Conflicts between the parties arise all the time, over valuations and much else. But it allows such firms to reach pools of capital that an old-fashioned family business would not have got its hands on."