Short answers, pulled from the story.
The upper class in The Age of Innocence consists of people who held the highest social status because their families had long held immense wealth and political power. This group differs from the newly rich middle classes that tend to dominate public life in modern social democracies.
Historically these individuals did not have to work for a living as they were supported by earned or inherited investments like real estate. Their status commonly derived from the social position of one's family rather than from one's own achievements or wealth.
Since the Second World War the term has come to encompass rich and powerful members of the managerial and professional classes as well. In many countries the term upper class was intimately associated with hereditary land ownership before this shift.
According to the book Who Rules America? by William Domhoff the top 1% of Americans own around 34% of the wealth in the U.S. while the bottom 80% own only approximately 16% of the wealth.
Bob Herbert of The New York Times wrote about modern American plutocrats as The Donor Class in 1998. He defined the class as a tiny group just one-quarter of 1 percent of the population that is not representative of the rest of the nation.