All I can really say about this is, WOW.
NASA's Kepler team has found loads of planet candidates to date (about 2300) by measuring variations in brightness in the star as the planet orbits. They've confirmed a bunch of them as well, so we know there are other planets out there. And just in case you think 2300 is a low number considering how many stars are in the sky, keep in mind that the Kepler telescope has been staring at a patch of sky about the size of your outstretched hand the entire time. I don't know about you, but I can't even see 2300 stars in that kind of an area, and the Kepler telescope has found that many planet candidates.
Now they've found one in its star's habitable zone - where liquid water can exist.
If you take a look at their news story, they show our solar system with the habitable zone defined - Venus is too close to the sun to be habitable, but Earth and Mars are both in the right area. This is why they've been sending robots to Mars to look for evidence of past water. Past water, because Mars' atmosphere is much too thin to retain enough heat to have present water, but it does have present ice in the form of its polar ice caps (as well as solid CO2, dry ice).
It remains to be seen whether this planet has an atmosphere which will hold in enough heat to maintain liquid water, but at least its orbit is in the right place for this even to be an option. Composition of some planets' atmospheres (mostly the early Jupiter-sized ones so far) has been studied using infrared absorption spectra, which is a whole level of nifty that I would need to study for a while to understand.