Below is an image pointing out clays and sulfates in the lower portion of Mount Sharp within Gale Crater. Mars Science Laboratory (MSL) Curiosity is set to land in Gale Crater on Monday 6 Aug at 0100ET. From there MSL will begin examining the surrounding terrain, with special emphasis on the clay minerals and sulfates because those were likely formed during a period when Mars was more hospitable to life as we know it.
We know this because clays and sulfates are formed in the presence of water. Orbital studies of Gale Crater have indicated that the water that once filled the crater was less acidic than in other areas previously studied on Mars. Water is more hospitable to life when it is less acidic, so scientists hope that examining the clays and sulfates formed in that water will tell them whether Mars was, or still is a conducive environment to life as we know it.
A lot of people think that Mars Science Laboratory was sent to Mars to find life, but this isn't the case. It is landing on Mars to determine if the conditions on Mars were or still are favorable to life. That doesn't mean MSL couldn't find life, it just means that it isn't searching for it.
So why does Mars Science Laboratory's mission matter? If we find out that Mars was or still is an environment that can sustain life, it will raise our hopes that it did once contain life. The mission will examine the layers in Mount Sharp, giving us a better understanding of the timeframe during which the water on Mars disappeared. If we know when the water on Mars disappeared we can better determine why and perhaps learn how to prevent such a catastrophe from happening to us on Earth!
When we determine how long ago water was abundant on Mars we can start narrowing down our search for life by only examining sediments and other material formed during those time periods since water is generally thought to be necessary for the development of life. So Mars Science Laboratory is key to understanding more about the past on Mars and in the solar system!