Your aim as mayor is to build and sustain a happy city. You have a limited budget with which to increase your city every year, and you have to keep your residents happy in order to increase the number of people who move in, which in turn will increase the money you receive in taxes. This means building enough roads, attractions and commercial/industrial districts, and paying attention to rising pollution, crime and traffic.
You are helped along the way by your assistant, Dr. Wright, a bizarre character based on Sim City series creator Will Wright.