From an urban design perspective, several differences and similarities exist between the municipalities. For instance, municipalities with 50,000 inhabitants or more are considered incorporated cities, while those with fewer than 50,000 are considered incorporated towns. An additional difference between the two is the amount of autonomy each municipality has, with cities provisioning their own services while towns typically depend on nearby cities for certain services. Regardless of this differentiation, all municipalities have a barrio called pueblo proper (English: 'town') which typically hosts that municipality's original Spanish settlement and is also typically that municipality's urban core. Municipalities with large populations, however, may have an urban core that consist of several barrios.
Other differences exist among the municipalities. Economic activity, for example, concentrates on the metropolitan areas surrounding the cities of San Juan, Ponce, Arecibo, Caguas, Mayaguez, Aguadilla, and Humacao, with most towns being commuter towns. Statistically, the municipality with the largest number of inhabitants is San Juan with around 400,000 while Culebra is the smallest with around 1,800. While in terms of territorial extension, Arecibo is the largest with around 125 mi2 and Cataño the smallest with around 4.8 mi2.
Because Puerto Rico was a Spanish colony until 1898, its system of local government bears more resemblance to that of the Hispanophone nations of the Americas than to local government in the United States and some other Anglophone countries. Thus, there are no first-order administrative divisions akin to counties, as defined by the United States Government; instead, Puerto Rico has 78 municipalities or "municipios" as the secondary unit of administration. For U.S. Census purposes, the municipalities are considered "county-equivalents." The municipalities are grouped into eight electoral districts, but these do not possess administrative functions. In 1991, the Autonomous Municipalities Act was passed, which slightly modified the rights and responsibilities of Puerto Rican municipalities with the aim of decentralizing control and improving government services.
Every municipality (except San Juan) also has an urban area made up of one or more barrios. When the urban area is made up of only one barrio, it is called "Barrio Pueblo". Some urban areas are made up of multiple barrios: Ponce's urban area, for example, is made up of 12 barrios. All of San Juan's barrios are urban barrios, and the municipality of San Juan is composed of urban barrios only - thus, the entire municipality of San Juan consists of one large urban zone only.