Mass is an intrinsic property of matter, whereas weight is a force that results from the action of gravity on matter. However, in everyday usage they are usually not quite different. When we buy goods "by weight", we are primarily interested in the amount of goods (the mass), not how hard they press down on the table (the force).
Units of weight are simply units of force. The SI unit of force is the newton (N), which can also be expressed in SI base units as kg•m/s².
The kilogram-force is a non-SI unit of weight, defined as the force exerted by a one-kilogram mass in Earth gravity. In English units, the pound has been used as a unit of force. The poundal also has been used, defined as the force necessary to accelerate a one-pound object at 1 ft/s². A poundal is about 1/32 of a pound.
A weighing scale is a device for measuring the weight of an object.