It's a game where you run a city. In the latest released incarnation, SimCity 4, this involved zoning for certain types of buildings (residential, commercial, or industrial), placing community and service buildings (schools, police, reward buildings, etc.), laying out transportation systems, directing funds, and of course, disasters. Also had some mini-games with the Rush Hour EP.
I'm one of those people who likes unadulterated political and societal planning, so it's well worth my money.