Curiosity, sometimes referred to as the Mars Science Laboratory, is an unmanned vehicle that is currently exploring the surface of Mars. Curiosity is much like the successful twins Spirit and Opportunity, being a multiwheeled vehicle able to navigate and explore over a large area, but is even larger, being the size and weight of an automobile.
Curiosity has six wheels, is nine feet long, has a nuclear battery and ten scientific instruments. The scientific instrument most often mentioned is a laser/spectroscope combination that will be able to vaporize small pieces of the Martian surface, and then analyze their chemical composition, including possible signs of complex organic molecules that could give evidence of past Martian life. The instrument that is now mostly in use is a "simple" camera, that has been broadcasting back color, high resolution pictures of the Martian surface.
Perhaps even more important than what Curiosity can do, is the technical ingenuity of getting it to the surface of Mars. Because it was much bigger (and complicated, and expensive) than past wheeled vehicles sent to the surface of Mars, it needed a different delivery method. This consisted of a series of methods, used to slow down the craft in the thin Martian atmosphere. From an initial velocity of 26,000 miles per hour, a heat shield, parachute and sky crane slowed Curiosity down to a leisurely 2 miles per hour when it reached its destination in Gale Crater. Since Mars is a dozen light minutes from earth, the craft had to make its final approach using only its flight programming, since there would have been nearly a half-hour delay in communicating instructions to the craft. The period right before the landing of the craft was anxious, since a slight miscalculation could have led to a 2.5 billion dollar crater on the ground. Luckily, however, the craft landed without the slightest problem.
As of the current writing, Curiosity has been moving around for ten days, and has not even begin to use its more fancy equipment. When it does, it will probably bring back information that will advance several fields of science greatly. Looking at another planet's composition will help us understand earth's own geochemical cycles, as well as astronomic questions of how planets are formed. Of course, the greatest reward would be good evidence for past or current life on Mars.
The public and media reaction to Curiosity's landing was greater than many had anticipated. The United States space program doesn't have anywhere near the popularity it did during the Apollo Program or even the beginning of the space shuttle era. Most missions have been too technical and too obscure to capture much mass attention. However, the Curiosity mission, perhaps due to how daring it was, did. Soon after it landed, and its first grainy pictures were transmitted back to earth, the NASA website got so many hits that it couldn't keep up. And in spite of a fairy busy news week, with a presidential election and The Olympics, Curiosity was a top story in the news. The success of Curiosity gave a boost of optimism to many people.
Curiosity has had a good run so far, and hopefully it will continue to produce good results. Also the success of the complex Curiosity landing procedure will encourage NASA to look at even more ambitious missions to Mars, including a manned mission.
http://www.nasa.gov/mission_pages/msl/ is the official NASA page for Curiosity.