The current mission to Mars involves a pair of robotic rovers that are known as the Mars Exploration Rovers (MER). Why are we sending robotic rovers rather than sending people like we did when we explored the Moon?
We aren't really at the point yet where we can send human beings to Mars. The first and most important reason for that hesitation is our track record -- different nations have sent more than 30 probes toward Mars, but fewer than one-third of those probes have survived the trip. It's not a very good track record, and certainly not one that would encourage us to replace those robotic probes with human beings, at least until we've improved the odds of success. ¬The second reason is cost. As we will see in a moment, it is currently costing about a million dollars per pound to design and deliver a robot to Mars, and robots don't have to worry about complicated things like life support systems. Nor do robots have to worry about coming home -- something that adds a great deal of weight to a mission. Nor do robots require a soft landing on the surface of Mars.
It would take a minimum of 100,000 pounds of vehicle, equipment, food and water to get a small team of people to Mars (each person, for example, will require 900 pounds (408 kg) or more of dehydrated food). At a million dollars a pound, that's $100 billion right there. And chances are that a manned mission would cost more per pound than a robotic mission because of the significant safety margins needed for human passengers.
The third reason is the engineering challenges. For example, to make a manned mission possible, one likely scenario is to produce fuel for the return flight from the Martian atmosphere. However, nothing like this has ever been attempted, and it would take a number of test missions to prove out the concept. Another big consideration is the cosmic radiation that astronauts would absorb during such a long mission, and how to block it. Much of this radiation is blocked on Earth by the Earth's magnetic field. Mars has no magnetic field.
So, we will not be sending humans to Mars any time soon. That leaves us with the option of sending robots instead. The MER robots are a manifestation of that philosophy.