- where Net Profit = Revenue - Cost
The profit margin is mostly used for internal comparison. It is difficult to accurately compare the net profit ratio for different entities. Individual businesses' operating and financing arrangements vary so much that different entities are bound to have different levels of expenditure, so that comparison of one with another can have little meaning. A low profit margin indicates a low margin of safety: higher risk that a decline in sales will erase profits and result in a net loss, or a negative margin.
Profit margin is an indicator of a company's pricing strategies and how well it controls costs. Differences in competitive strategy and product mix cause the profit margin to vary among different companies.
Profit margin is frequently confused with markup. It's not uncommon for entrepreneurs to erroneously claim profit margins over 100%. Most likely these entrepreneurs are referring to the markup on a product as a percentage of product cost.
See also 
- Profit Margin - an online profit margin calculator.