A basic cornerstone of the capitalist business model. Provides incentive for inventors to create products, and for investors and marketeers to manufacture and distribute them.

Just to make it explicit, the profit margin is the calculated as:

                 gross - expenses
profit margin = ------------------
                     gross

Depending on what sort of business you're in, a good profit margin is anywhere from 5% (in groceries, for example) to 100% (selling drugs). It sure as hell better be positive, though.