Curiosity is not on a life detection mission: the rover isn't actually looking for life. The rover doesn't have the ability to detect life even if it was there. What it is looking for, is the ingredients of life.
The Mars Science Laboratory takes this Curiosity Rover with this incredible set of payload instruments to figure out if Mars ever could have supported microbial life. It's looking for a place where micro organisms, little tiny single-cell organisms could have lived. That requires a source of energy and water because all life as man knows it is associated with water, and then the rover also needs to find a source of carbon to show that Earth-like life would have been possible on Mars.
Curiosity is going to land at Gale crater where it will be climbing a mountain. In fact, one of the first things researches will see from images of Curiosity’s first day on Mars is this giant mountain in front of the rover just waiting for it a few miles away. And in that mountain there's a stack of layers, and like turning the pages of a book Curiosity will explore these layers and look at them in terms of whether or not they preserved evidence for ancient habitable environments.
If one thinks of Spirit and Opportunity as robotic geologists Curiosity goes one step further. It's not only a robotic geologist, but a robotic geochemist. The research needs a bigger rover this time around because it has ten science instruments and two of them fit inside of the belly of the rover. It has state-of-the-art laboratories to do very detailed geochemical analysis of the rocks and soils on Mars and the atmosphere as well.
The rover will have to feed those instruments by getting samples of rock with a big robotic arm and a drill on the end of it. It is armed with cameras and other detectors to monitor the weather and other environmental challenges. Researchers need to make those measurements in order to know that — if life had evolved on Mars — would this be the kind of place where microorganisms could have lived.