Brownstone is a brown Triassic sandstone which was once a popular building material. While brownstone is often popularly associated in the United States of America with New York City and Chicago, the stone was used widely around the world before losing popularity around 1900 in part due to rapid failures of carved surface details in the weathering process. The quarries used for the early brownstones of New York City were in New Jersey, and in the Connecticut River area. Brownstone with qualities approaching granite is currently being imported from Germany.
In New York City a "brownstone" is understood to be a terrace or rowhouse clad in brownstone. These brownstone apartments typically have stairways which lead from the sidewalk to a second-floor apartment entrance, a design originally intended to avoid bringing in the mud and horse droppings commonly found at street level, a problem that existed when these apartments were built and horses roamed the streets. New York City brownstones tend to be found in certain older neighborhoods, which are perhaps most common in Brooklyn. For example, the neighborhood of Bedford Stuyvesant has the largest inventory of brownstones in the entire City of New York, followed closely by Park Slope. Many brownstones have been renovated in recent years, leading to (and/or as a result of) gentrification in areas like Park Slope, Bedford Stuyvesant and Fort Greene. On the popular 1980's-'90s American television program The Cosby Show, the affluent Huxtable family, the show's central characters, lived in a Brooklyn brownstone.
Excerpt from Wikipedia