In the United States, a county is a political and geographic subdivision of a state, usually assigned some governmental authority. The term “county” is used in 48 of the 50 U.S. states. The exceptions are Louisiana and Alaska, where the functionally equivalent subdivisions are called, respectively, parishes and boroughs. Numerous consolidated city–counties exist throughout the U.S. in which a city has merged with its county to form one unified jurisdiction with the governmental powers of both entities.
The U.S. federal government uses the term “county equivalent” to describe administrative or statistical areas that are comparable to counties. The number of counties per state ranges from the 3 counties of Delaware to the 254 counties of Texas.
(Source: wiki – http://en.wikipedia.org/wiki/County_%28United_States%29)
Here is a list of 10 most populated counties in the United States
10QUEENS COUNTY, NEW YORK
Queens is the easternmost of the five boroughs of New York City, the largest in area, and the second-largest in population. The borough of Queens has been coterminous with Queens County since 1899. The county is now the second most populous county in New York State.
(Source: wiki - http://en.wikipedia.org/wiki/Queens)
09DALLAS COUNTY, TEXAS
Dallas County is a county located in the U.S. state of Texas within the Dallas–Fort Worth–Arlington metropolitan area (colloquially referred to as the Dallas/Fort Worth Metroplex). As of the 2010 census, the population was 2,368,139 and is now the ninth most populous county in the United States. Its county seat is Dallas, which is also the largest city in the county, the third-largest city in Texas, and the eighth-largest city in the United States.
(Source: wiki - http://en.wikipedia.org/wiki/Dallas_County,_Texas)
08KINGS COUNTY, NEW YORK
Brooklyn is the most populous of New York City's five boroughs, with about 2.5 million people, and the second-largest in area. Since 1896, Brooklyn has had the same boundaries as Kings County, which is one of the most populous county in New York and the second-most densely populated county in the United States
(Source: wiki - http://en.wikipedia.org/wiki/Kings_County,_New_York)
07MIAMI-DADE COUNTY, FLORIDA
Miami-Dade County (commonly called Miami, Miami-Dade, Dade County, Dade, Metro-Dade or Greater Miami) is a county located in the southeastern part of the state of Florida. As of the 2010 census, the county had a population of 2,496,435, making it the most populous county in Florida and the seventh-most populous county in the United States. It is also Florida's third largest county in terms of land area, with 1,946 square miles (5,040 km2). The county contains approximately half of the Miami metropolitan area's population and several of its largest cities. The county seat is Miami.
(Source: wiki - http://en.wikipedia.org/wiki/Miami-Dade_County,_Florida)
06ORANGE COUNTY, CALIFORNIA
Orange County is a county in the U.S. state of California. Its county seat is Santa Ana. As of the 2010 census, its population was 3,010,232, making it the third most populous county in California, behind Los Angeles County and San Diego County. The county is famous for its tourism, as the home of such attractions as Disneyland and Knott's Berry Farm, and several beaches along its more than 40 miles (64 km) of coastline. It is known for its political conservatism – a 2005 academic study listed three Orange County cities as being among America's 25 "most conservative."
(Source: wiki - http://en.wikipedia.org/wiki/Orange_County,_California)
05SAN DIEGO COUNTY, CALIFORNIA
San Diego County is a county located in the southwestern corner of the State of California. San Diego County has 70 miles (110 km) of coastline. Most of the county has a mild Mediterranean climate to semiarid climate, though there are mountains that receive frost and snow in the wintertime.
(Source: wiki - http://en.wikipedia.org/wiki/San_Diego_County)
04MARICOPA COUNTY, ARIZONA
Maricopa County (/ˌmærɨˈkoʊpə/ MARR-i-KOH-pə) is a county located in the south-central part of the U.S. state of Arizona. As of the 2010 census, its population was 3,817,117, the most populous in the state, and ranks fourth among the nation's counties and is greater than the population of 23 states. Maricopa County is also one of the largest counties in the United States by area having a land area greater than that of seven states. The county seat is Phoenix, which is Arizona's largest city and capital.
(Source: wiki - http://en.wikipedia.org/wiki/Maricopa_County,_Arizona)
03HARRIS COUNTY, TEXAS
Harris County is a county in the U.S. state of Texas. As of the 2010 census, the population was 4,092,459, making it the most populous county in Texas and the third most populous in the United States. Its county seat is Houston, the largest city in Texas, and the principal city of the Houston–The Woodlands–Sugar Land metropolitan area.
Harris County was founded in 1836 and is named for John Richardson Harris, an early settler of the area.
(Source: wiki - http://en.wikipedia.org/wiki/Harris_County,_Texas)
02COOK COUNTY, ILLINOIS
Cook County is a county in the United States state of Illinois, with its county seat in Chicago. It is the second most populous county in the US after Los Angeles County, California. The county has 5,231,351 residents,[1] which is 40.6 percent of all Illinois residents. Cook County's population is larger than that of 29 individual U.S. states and the combined populations of the seven smallest states.[
(Source: wiki - http://en.wikipedia.org/wiki/Cook_County,_Illinois)
01LOS ANGELES COUNTY, CALIFORNIA
Los Angeles County, also known as L.A. County, officially the County of Los Angeles, is a county in the U.S. state of California. As of the 2010 U.S. Census, the county has a population of 9,818,605, making it the most populous county in the United States. Los Angeles County alone is more populous than 42 individual U.S. states. The county seat is the city of Los Angeles, the largest city in California and the second-largest city in the United States.
(Source: wiki - http://en.wikipedia.org/wiki/Los_Angeles_County,_California)