Rosetta is a European space probe which has been sent to comet Churyumov-Gerasimenko, where it will eventually attempt to deploy a special lander. It is named after the Rosetta Stone.
The European Space Agency has a long history of work in close comet approach missions. The Giotto probe was first launched in 1986, and made a close approach to Halley's Comet. NASA and the ESA intended to collaborate on another cometary mission during the early 1990s, using a spacecraft modeled after the Mariner probes, but NASA withdrew due to cost factors and the ESA was left to go it alone. Eventually this became the Rosetta probe.
The Rosetta probe carries six basic instruments. Five of these are sensors on the probe itself, including a UV spectrograph (to identify noble gases on the comet), and optical and infrared camera, a visible and infrared spectrograph (to search for other chemicals), a microwave scanner, which will pick up signs of water and ammonia, if those are present; and a radar-mapping device which will map the interior of the comet. Other instruments will scan free-floating molecules, dust, and the solar wind. The probe is also carrying Philae, a lander which will anchor itself to the surface of the comet and carry out a range of studies using its own onboard instruments
Rosetta was originally scheduled for launch in 2003, and an approach to comet Wirtanen in 2011. However, the launch was delayed until 2004, which required altering the target to Churyumov-Gerasimenko. This comet was discovered by Soviet astronomers in the 1960s. with an orbit of about 6.6 years (typical of a population of comets whose orbit takes them out roughly as far as that of Jupiter). Churyumov-Gerasimenko's orbit has been substantially altered by Jupiter's gravitational influence on several occasions during the past two centuries.
Rosetta will reach Churyumov-Gerasimenko in 2014. In the meantime, it has made flybys of Earth, Mars, and asteroids Steins and Lutetia. Lutetia's surface is battered and scarred, and it is believed that this asteroid is billions of years old. The spacecraft also made a close approach to a small mysterious object known only as P/2010 A2, which was discovered by the LINEAR telescope in early 2010 and initially designated a comet. Images from Rosetta confirmed that the object was actually an asteroid, and that the stream of dust behind it (resembling a comet's telltale trail through the sky) was actually the debris released by impact with a smaller, meteor-like event.
Updates to the Rosetta mission can be found on the European Space Agency's official project website.