Laws that only certain buildings can be built in certain areas for certain uses. A lot of American cities have laws that essentially force there to be single family homes with driveways, yards, and looping roads. Some zoning laws say buildings have to be constructed a certain distance apart from one another. Some will dictate all buildings in a certain area need parking lots of a certain size.

Typically zoning will split things between three things: commercial, residential, and industrial. Without zoning, buildings just go anywhere, regardless of use.

Houston is currently the largest city in America without formal zoning laws and it shows. Suburbs within walking distance of petroleum refineries.