The Kepler Mission is part of the rise of next generation super telescopes that are in the feverish race to locate an exoplanet that closely resembles Earth. With the the staggering number of stars much like our own star, in our galaxy alone, there is a strong enough argument to be made to be searching for planets much like our own.
Now whether these planets truly exist is still being determined. This is exactly what Kepler is doing right now. The Kepler team hypothesize that they will have data within three years that shows hundreds of exoplanets that resemble our own terrestrial Earth. The other key is to find planets within the habitable zone.
A simple way to define the habitable zone is to use the analogy of "Goldilocks and the Three Bears". Kepler will not only find terrestrial planets that match our size but their distance from their respective stars. Our Earth is within the "Goldilocks" zone in our solar system, not too far, not too close. Kepler will determine this "Goldilocks" zone for the exoplanets it finds.
Kepler is already sampling a large portion of stars in a part of the milky way much like our own. This sample should give us some good returns on terrestrial planets. Once this sample is established there really is no need to get a bigger sample. The next logical step would be to launch the very expensive terrestrial planet finder.
This mission would have a resolution beyond our wildest imaginations. We would be able to then image these terrestrial planets and begin true research into whether a terrestrial exoplanet harbored life.
Kepler is establishing the odds, and the Terrestrial Planet Finder, once launched, would find the rare ace.