You'll have to forgive me, but I'll be flipping back and forth from the metric system (the system most of the world uses) and the Imperial system (that which Americans refuse to let go).
Mass is more or less defined as the amount of material an object has. More specifically, it is a measure of its inertia (which I will not go into here).
On the other hand, weight is a measurement of a mass muliplied by the gravitational pull another (usually larger) body of mass has.
Typically, mass is measured in kilograms, while weight is measured in pounds. The problem is, of course, that the kilogram is a unit of the metric system, and the pound is Imperial. These are two totally different measurment systems. But, most people don't know the counterparts in the opposing system (weight in metric, and mass in Imperial); (I'm sure there are a couple of you who read this who do, though!!!)
So, for now, here on Earth, we know as 1 kilogram being roughly 2.2 pounds.
So let's say I had three kilos of a product (if it makes it any easier for those of you who use Imperial, let's say it's three kilos of cocaine, as you've seen enough cop shows to know approximately how big that is.) and I took it to the moon. I still have the same amount of material, right? So therefore, I still have 3 kilos (once again, think cocaine if it helps you out). But, because the mass of the moon is approximately 1/6th that of Earth, its gravitational pull is also about 1/6th, which then means that its weight has changed. On Earth, that three kilos is about 6.6 pounds. On the moon, it is only about 1.1 pounds. I still have the same mass/the same amount of material (and it's probably worth the same amount, too, you little addicts!!!), but because the gravitational pull is less, so is its weight.
Now, if you take that three kilos into a part of outer space where there is no gravitational pull, you are going to come up with a weight of zero. Nothing, zilch, nada, nichts, zippo!
Remember, weight is a measurement of a mass multiplied by the gravitaional pull of another object.
Simply: Wt = m * g
On the surface of Earth, gravity is measured as 32 feet per second(squared) [32ft/sec²]. This formula is actually a formula of acceleration. To put things simply: things do not fall towards the earth at a constant rate; as they fall, they get faster and faster. The longer it falls, the faster it gets! This is why if you jump out of a two story window you would be less injured than if you jumped out of an airplane without a parachute 30,000 feet up in the air. If you fell without going any faster, you wouldn't even need that parachute!
For those who are curious to know, in the metric system, weight is measured in Newtons, which is the same as Force (Force = mass * acceleration == weight = mass * gravity, and gravity is a measure of acceleration, so, they're the same).
And truthfully, I don't remember what the Imperial unit of mass is (I keep thinking 'stone' but I have doubts). Honestly, I did know what it was at one time!!!