Mars Global Surveyor is a global mapping mission, examining the entire planet - from the ionosphere (an envelope of charged particles surrounding Mars) down through the atmosphere to the surface, and deep into Mars' interior. As part of the larger Mars Exploration Program, Mars Global Surveyor performs atmospheric monitoring for sister orbiters during aerobraking, and it helps rovers and lander missions by identifying potential landing sites and relaying surface telemetry.
Goals: Mars Global Surveyor contributes to meeting the four main science goals of the Mars Exploration Program:
1) Determine whether life ever arose on Mars
2) Characterize the climate of Mars
3) Characterize the geology of Mars
4) Prepare for human exploration of Mars
(taken from http://mars.jpl.nasa.gov/mgs/science/)