The Brownstone is the type of architecture most commonly found in the Back Bay. Brownstone is actually a type of sandstone sedimentary rock. It is abundantly found in the New England area and made for strong and convenient building material in the 1800's when the Back Bay was filled in with landfill.
The most popular type of brownstone in the Back Bay is the "Queen Anne" style brownstone. It originated in the mid 1800's England. Boston & England have very similar architecture (especially Boston's Back Bay), and many homes in both Boston & England were developed in the Victorian Era.
The brownstone is also found in other parts of Boston, as well as other American cities such as New York, Philadelphia, and Chicago. Back Bay brownstones were once single family homes, and most have now been chopped into condos & apartments.