As a mayor, it is your job to manage your city. It starts off empty, but will grow. You can zone parts of your city for residential, commercial & industry. In addition to zoning, you can build roads, parks, trains, water pipes, power plants and much much more. If you are successful, your city will grow and prosper. If you fail you will bankrupt the city and get thrown out of office.
When announced in 1997, Simcity 3000 was originally going to use a full 3D engine to allow players to explore their own cities right down to street level, using a first person perspective. However, technology issues forced the developers to scrap the engine and return to the classic isometric 2D style.