I believe that Romney is officially "on leave" from Bain Capital, and is still a major shareholder. With that said, Bain Capital's business is to buy distressed business with other people's money, return those businesses to health, and reward those other investors with significant rewards when the businesses are sold, or taken public.
All Presidents are required to put their business holdings into a blind trust. Of course, it's very unlikely that the blind trust is going to make major changes in the holdings in that portfolio. Some presidents have pursued policies that boosted the value of their holdings. Some presidents have pursued policies that hurt their personal wealth, sometimes in an attempt to avoid charges of favoritism.