Kepler will be in search of orbs like Earth: "rocky planets that orbit sun-like stars in a warm zone where liquid water could be maintained on the surface," according to NASA. In other words, it's looking for places where life might be possible.
The business end of Kepler is its photometer, a 0.95-meter telescope with a field of view of 105 square degrees--"comparable," NASA says, "to the area of your hand held at arm's length." Such a relatively large field of view for an astronomical telescope is needed so that Kepler can monitor the brightness of 100,000 stars.
Liftoff for Kepler is scheduled for 10:49 p.m. EST on Friday.