Comet Hartley 2, or 103P/Hartley, was discovered in 1986 (by Malcolm Hartley) and has a period of about 6.5 years. On October 20, the comet made its closest approach to Earth at 11 million miles (46 times the distance to the Moon, not too close!). It was visible in October (and still is) through binoculars or a small telescope (it wasn't very impressive, however).
One surprise already is that the probe detected a surprisingly large release of cyanogen (cyanide) gas from the nucleus of the comet. While cyanogen is a common component of cometary nuclei, the surprise was the amount, given the relatively small 2 km diameter of the comet, and the fact that dust wasn't also emitted - it was pure gas. Normally, you'd expect both.
Cyanogen gas (blue) vs dust (black) from comet Hartley 2
Anyway, it should be neat to see the close-up images. Here's NASA's website for the mission (just in case the nightly news is more concerned with important events of the day like what Snookie did or what Lady Gaga is wearing).