From the European Space Agency:
"Comets are considered the primitive building blocks of the Solar System, and likely helped 'seed' the Earth with water, and maybe even life. By studying the nature of the comet's dust and gas, Rosetta will help scientists learn more about the role of comets in the evolution of the Solar System.
Rosetta will be the first mission ever to orbit a comet's nucleus and land a probe on its surface. It will also be the first spacecraft to fly alongside a comet as it heads towards the inner Solar System, watching how a frozen comet is transformed by the warmth of the Sun.
Rosetta is the first space mission to journey beyond the main asteroid belt and rely solely on solar cells for power generation, rather than the traditional radio-isotope thermal generators. The new solar-cell technology used on the orbiter's two giant solar panels allows it to operate over 800 million kilometres from the Sun, where sunlight levels are only 4% of those on Earth."