With two exceptions,[a] their areas match the 49 census divisions Statistics Canada has for Ontario.

The first three are types of municipal government but districts are not—they are defined geographic areas (some quite large) used in many contexts.

(Lower-tier municipalities are generally treated as census subdivisions by Statistics Canada.)

In some cases, an administrative division may retain its historical name even if it changes government type.

Some single-tier municipalities of this type (e.g., Toronto, Ottawa, Hamilton, Greater Sudbury) were created where a former regional municipality consisted of a single dominant urban centre and its suburbs or satellite towns or villages, while others (e.g., Brant County, Chatham-Kent, Haldimand-Norfolk, Kawartha Lakes, and Prince Edward County) were created from predominantly rural divisions with a collection of distinct communities.

They generally provide the following services: maintenance and construction of arterial roads in both rural and urban areas, transit, policing, sewer and water systems, waste disposal, region-wide land use planning and development, as well as health and social services.

Although districts do still contain incorporated cities, towns and townships, they do not have an upper-tier county or regional municipality level of government, and are largely composed of unorganized areas.

Much of Northern Ontario is sparsely populated, so a county government structure would not be an efficient or cost-effective method of administration.