There have in fact been several Mars rovers, rather than one specific spacecraft sent to Mars. These spacecraft are used to explore the surface of Mars for the purpose of gathering scientific data of interest to people on Earth. Thus far, only the United States has successfully launched and landed Mars rovers, with several missions to date.
A rover is designed to land on the surface and traverse it, gathering valuable information. Roving technology can be used to explore a variety of celestial bodies in addition to Mars, and robots have also been utilized to gather data about hostile environments such as the bottom of the ocean. A Mars rover has a rugged design which allows it to survive on the surface of Mars, along with an autonomous computer which directs the rover, and communication methods to send images and data back to Earth.
There are several reasons to send out a Mars rover. First of all, rovers provide much more information than orbiting spacecraft, allowing researchers to learn more about the climate of Mars and what the terrain is like. Unlike human astronauts, a Mars rover can survive just fine on the surface of Mars, and it can endure the trip to the fourth planet. The National Aeronautics and Space Administration (NASA) has stated that it is using rovers to learn more about the environment on Mars and the history of the planet, and to prepare for potential human exploration.
The earliest attempt at a Mars rover was made in 1971, when two attempts were made to send robots to Mars, and both failed. In 1997, the Sojourner was launched to explore Mars. Although it landed successfully, it failed within a few months, much to the frustration of scientists. Another launch in 2003 resulted in the 2004 landings of the Mars Spirit and Mars Opportunity, two rovers which were gathering data as of 2009.
These robots have sent back some astonishing images of the surface of Mars which have revealed a great deal of information about the terrain of the planet. They have also taken geological samples and provided a wide array of useful scientific data. NASA periodically releases information and images of interest from its Mars rover projects, to keep the public interested in the sciences and to promote the Mars rover program. Scientific research on Mars is quite expensive, and NASA relies heavily on the support of the public to receive continued government funding.