"Aristocracy is always cruel."
Wendell Phillips (1811-1884)
In order of importance, the "peerage" -- or titled aristocracy - is ranked duke, marquis, earl, viscount, and baron. Having such a title automatically granted the owner a seat in the House of Lords, the upper house of Parliament. (The House of Commons consisted of elected members who were required, however, to own a certain amount of property).
The peerage was a landowning class whose income came from the rental of their property, often thousands of acres, to tenant farmers. The eldest son was eligible to inherit the estate -- and the peerage -- from his father. An aristocrat continues to hold the title he inherits and doesn't move "up" the scale of nobility.
The children of peers are commoners. Only after the death of the father would the oldest son become an aristocrat, inherit the title and take a seat in the House of Lords. Until that time, the eldest son would use his father's second-most-important title. Thus, the Earl of Cumnor, who would be addressed as "Lord Cumnor" has a son who is addressed as "Lord Hollingford."
A countess was the wife of an earl; typically a married woman's status depended upon the status of her husband.
From Wives and Daughters, Part One, Chapter I
...The little straggling town faded away into country on one side close to the entrance-lodge of a great park, where lived my Lord and Lady Cumnor 'the earl' and 'the countess', as they were always called by the inhabitants of the town; where a very pretty amount of feudal feeling still lingered, and showed itself in a number of simple ways, droll enough to look back upon, but serious matters of importance at the time. It was before the passing of the Reform Bill, but a good deal of liberal talk took place occasionally between two or three of the more enlightened freeholders living in Hollingford; and there was a great Tory family in the county who, from time to time, came forward and contested the election with the rival Whig family of Cumnor. One would have thought that the above-mentioned liberal-talking inhabitants would have, at least, admitted the possibility of their voting for the Hely-Harrison, and thus trying to vindicate their independence But no such thing. 'The earl' was lord of the manor, and owner of much of the land on which Hollingford was built; he and his household were fed, and doctored, and, to a certain measure, clothed by the good people of the town; their fathers' grandfathers had always voted for the eldest son of Cumnor Towers, and following in the ancestral track every man-jack in the place gave his vote to the liege lord, totally irrespective of such chimeras as political opinion...