Hi Leticia,
Weight is a measurement of the gravitational force acting on an object and its SI units are Newtons (N). Mass is how much matter an object has and its SI units are kilograms (kg). For more units of mass visit :http://en.wikipedia.org/wiki/Category:Units_of_mass
Gross or total weight includes the product and its packaging like for example an unopened can of soup. Net weight refers to the weight of the product itself (the soup), not including the weight of packaging or other materials (the can). Often printed on packaging like an aluminum can, the unit given for net weight will be ounces (oz) or grams (g). Net weight is a misleading term as most manufacturers will actually be giving you the net mass.
It is very simple to calculate Net Weight:
Net weight = gross weight - weight of packaging

I hope this helps clear up your understanding of net weight. Feel free to send in a specific question for further understanding.

Janice