This isn't particularly surprising, for those who have been following events, but here is a little bit of history and background, and why this is actually pretty exciting:
Why we don't expect water
First of all, the moon is pretty small. It's big for a moon, especially compared to Earth, but in terms of being able to hold onto an atmosphere, there's just not enough mass. Being able to hold onto volatiles (gases, light molecules) depends on two things: how strong the gravity is, the temperature. Temperature matters because the hotter the gas is, the faster its molecules are moving, and the easier it is for them to reach escape velocity. The moon hasn't got the mass to hold onto things like water, given its location in the solar system (closer to sun = hotter.) Since there is no atmosphere, water would remain gaseous (instead of being able to freeze out or be liquid - you need pressure for that, for water at our distance from the sun - and the gas would escape because it's too hot & not enough gravity.
So, no water on the moon. However, the Apollo missions did bring back rocks that had trace water in them. The problem was, it was a real possibility that those rocks had been contaminated by water from Earth.
Why we might expect water
Water on the moon could in theory be created in situ (right there). If you have rocks hanging around on the moon (which it has plenty of), and those rocks contain oxygen - not the gas, but minerals that have oxygen in them - then it's possible that as the solar wind (protons, or hydrogen) smacks into those rocks it could knock the oxygen off. Then, with oxygen floating around (briefly) and hydrogen from the solar wind, you are likely to form a water molecule.
Ok, so now you have a water molecule. You still have the problem that it's too hot and there isn't enough gravity for the moon to hold onto it. The thing is, though, that the water won't just immediately fly away. Instead, it may wander around near the moon's surface, waiting for just the right energy kick to get it going. Just because the average gas molecule (and more than just that) are going too fast to hold on, doesn't mean that all molecules are going too fast. Some of them will be slower, and will stay for a brief while until they get knocked faster by a solar wind particle or something. So there should be small (trace) amounts of water around on the moon, in the top part of the soil, before they wind up wandering into space. This is what India's mission Chandrayaan-1 found, confirming other less-clear mission findings.
This doesn't help with persistent water though. If we want to use water for a manned mission, we really need more than what Chandrayaan-1 found. Let's go back to our water molecule wandering around near the moon's surface.
Every now and then, a water molecule will find its way to the polar regions, and land in a crater. What's special about these craters? The main thing is that near the pole, they are in perpetual night. Since they don't get any sunlight, they remain very, very cold (-238C, or -396.4F). Once our water lands in one of these craters, then, it will actually freeze despite the lack of atmosphere. In this way, we can slowly over time build up a reservoir of frozen water.
LCROSS Mission
Basically this mission involved crashing a probe into the moon, which then kicked up a big dust plume. An orbiting spacecraft then measured the water content of the dust kicked up. The answer is: Yes, there is water!
"Multiple lines of evidence show water was present in
both the high angle vapor plume and the ejecta curtain created by the
LCROSS Centaur impact. The concentration and distribution of water
and other substances requires further analysis, but it is safe to say
[the crater] holds water."
ARRR!
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