The difference between mass and weight is that mass is constant and fixed, while weight varies with location because weight is a measure of the force that gravity has on an object. Your mass on the Moon, the Earth and on Jupiter is the same; however your weight would change based on the amount of gravity being exerted on you. Earth’s gravity is -9.8 meters per second squared. When you step on your bathroom scale, your mass is exerting a force on a spring that is calibrated to the Earth’s gravitational field (-9.8 meters per second squared) in order to give you a weight reading.
Of particular note, is that the gravitational force exerted on the body can vary as much as 0.5% depending upon your location on the earth. In the image on the right, the different colors represent changes in the force of gravity. Wikipedia has an excellent chart that details the force of gravity in different cities around the world. In order for scientists to determine mass with better accuracy, a balance measure is used. A traditional balance is a beam with arms of equal length each holding a scale. The product of unknown mass is placed on the scale and measured against objects with known masses. This technique overcomes the variation in Earth’s gravitational field because both the known and the unknown masses are being equally acted upon.
To summarize mass versus weight, mass is constant and is a measure of how much matter an object contains, whereas weight changes according to the gravitational force exerted upon it.