Short answers, pulled from the story.
The American upper class is estimated to constitute less than 1% of the population. A Gallup Poll from 2001-2012 found that 98% of Americans identified with one of five other class terms, with 48-50% identifying as middle class.
The top 1% of Americans own around 34% of the nation's wealth, while the bottom 80% own only approximately 16%. This distribution is highlighted in William Domhoff's book Who Rules America? as the primary indicator of upper-class influence.
The Donor Class is a term coined by Bob Herbert of The New York Times in 1998. He defined it as a tiny group, just one-quarter of 1% of the population, whose political donations buy access to decision-making far beyond what their numbers would suggest.
In Great Britain and Ireland, the upper class traditionally comprised the landed gentry and aristocracy with hereditary titles. The vast majority of post-medieval aristocratic families originated in the merchant class and were ennobled between the 14th and 19th centuries through intermarriage with the old nobility.
After Irish independence in 1922, the upper class all but vanished in the Republic of Ireland. Aristocratic titles within the Peerage of Ireland granted by the British monarch carry no recognition in the Irish Constitution, and contemporary Ireland is generally perceived to have a two-tier class system of working class and middle class.
In the early 1980s, the average net worth of individuals on the Forbes 400 list was $400 million. By 2004, the average had risen to $2.8 billion, and Forbes counted 374 US billionaires that year, reflecting a dramatic increase since then.