what really is non controlling interest and profit attributable to non-controlling interest

It's not a big item on balance sheet or income statement most of the time, but what really is non-controlling interest? Most sites would say it is shareholding in a company of 50%, but really, who are these minority shareholders, retail investors, institutional investors ? Why do we need a separate line for "profit attributable to non-controlling interest"? More importantly, how do they affect equity investors? When we do stock valuations, and say look at ROE, do we use "profit attributable to owners of the parent" (i.e. profit excludes that attributable to non-controlling interest) divide by "total equity excl. non controlling interest" ?