Net Debt is the sum of all short term debt, and notes payables, Long Term debt and preferred equity minus the total cash and equivalents and short term investments for the most recent reporting period.
Net debt is the level of debt remaining assuming all cash and equivalents were used to immediately pay of debt. Please note that a negative net debt number means more cash than debt (i.e. net cash).
Some industries may have more net debt than others; therefore, investors often compare a company's net debt to others in the same line of business. It is usually considered as a ratio versus the total assets of the company.