Mass is the amount of matter present in any given chunk (i.e. size/volume) of space. In mathematical terms, m=F/a (i.e. mass = Force divided by acceleration). This equation is obtained by manipulating Newton's Second law, F=ma.
An object's mass can be measured by first finding the weight of the object, the acceleration due to the gravity of the earth (~9.8m/s^2) and then using "Newton's Second Law" (F=ma, i.e. Force=mass x acceleration) to solve for the mass.
Because the most often used process of finding the mass of an object requires weighing it, people often confuse the mass of an object as being equivalent to the weight of an object. This is NOT true since weight is a "force", not a quantity of mass. if it where true, then Newtons famous equation, F=ma, would instead be F=m. Which is incorrect.
This confusion is apparent in the European us of Kg (mass unit) for measuring weight. When if fact the proper unit is "Newtons" if they are wanting to find the Weight of the object. However this system still works because the weight and the mass are proportional since the acceleration due to the gravity of the earth does not change.