Mass and weight are usually used interchangeably in common conservation; in the natural sciences, however (and especially in physics), these refer to two specific, different properties of matter. In essence, mass is an estimate of the amount of matter in an object, and is equivalent no matter where the object is located; however, weight is an estimate of how strongly the object is attracted by gravity, and therefore an object will have more weight on the surface of the Earth than, say, on the surface of the Moon (or, for that matter, in Earth's orbit, where it is essentially weightless).
In everyday terms, there is no practical difference between weight and mass, because on the surface of the Earth, all objects have weight, and we describe this weight in largely equivalent terms to mass. Technically, however, weight is defined somewhat more specifically, as the amount of force exerted on an object by gravity. In our case, this definition refers to the force exerted on us, and our various possessions, by the gravity of the Earth. However, were we to move to a location with weaker gravity, we would weigh less: a 150-pound man on Earth would weigh 25 pounds on the Moon, and nothing in orbit.
Weight is measured in pounds (lbs) and, in metric terms, in newtons. This introduces a mild complication because people who commonly use imperial measurements in daily calculations (such as Americans and many Canadians) typically use a weight measurement but also mean to refer to the mass of an object, whereas people in metric regions (such as Europe) usually use kilograms, a unit of mass, when they also mean to refer to the weight of an object. One newton is worth slightly less than a quarter-pound.
In contrast to weight, mass is a measure of how much physical matter is contained in the make-up of an object. In one sense, therefore, mass is a much simpler and more objective measurement: weight varies depending upon the source of gravity, but mass is always the same. Technically, mass has an equally complex definition, however; mass is specifically defined in physics as the extent to which an object will remain immobile under a force applied to it. Expressed all in metric units, a one-kilogram object is an object which can be moved at one metre per second (or 3 feet per second) through the application of a force of one newton (or one quarter-pound).
Mass is measured in pounds in imperial measurements - one pound in mass is equal to one pound in weight at the Earth's surface; elsewhere, they are technically distinguished by differentiating them into two systems, called pound-mass for mass, and pound-force for weight. In regions which use the metric system, mass is always measured in kilograms, which are equivalent to about 2 pounds in mass.