In the era of the Napoleonic Wars, particularly in England, London was becoming a fast-paced society which was quite different from any other of the big cities. The English had its own system of rank and society that would create a sense of tension among the so-called upper and lower classes, not to mention a huge difference in how society perceived you.
In order to belong to a class, you either had to have
A) Precedence (your family is part of the royals), or an aristocrat (peers, baronets).
B) Have enough money in the family plan to come up with a dowry big enough to tempt a potential husband with a higher income to join the two families together in solid financial freedom (Upper-middle classes)
C)Belong to a guild, be learned and presented in society for your deeds and make some income out of it (middle class)
D) Be less fashionable and live more moderately whilst still enjoying a less rich but more practical living (lower middle classes)
E) Be a servant, or serve in some fashion for a small income. Not become known in society (Lower classes)
F) Belong to the worst poverty-stricken class and perhaps be abandoned by society, or sent to a workhouse (the poor)
In the story, the families are basically upper-class people who have lost their money. That does not mean that they stop being upperclassmen, since their still have precedence and peerage with their name and family history. In turn, they have to keep up appearances in order to feed their egos, or their need to network and save their finances.
It is often explained this way:
The aristocracy and the royals were a class apart. They, of course, are historical families that come with generations of wealth acquired by their ancestors.
You can be made a baron, or inherit a baronet, which is the lowest rank within the aristocracy, by your own merits or by friendship with a member of the royal family. Being a peer, like Sir Walter, meant that he likely came from peerage as well, and that he had rights and privileges that the middle class would not have.
The upper classes-
This class was mostly made of the aristocrats and peers, but later on in the late 1800's it will begin to fill with merchants and business owners, even naval officers like CPT Wentworth
The middle classes-
The middle classes are divided into the upper middle and the lower middle classes. The upper middle classes are our modern lawyers (QC's), and other learned men that have achieved class and enough income from their learned professions. The lower classes are people who work as tradesmen, or other smaller jobs but that may still have enough to live a calm yet unfashionable life because they are not classed in a higher strata.
The LOWER classes
These are basically the servants, and servile individuals who have to make a living on a small income.
The poor and the lower classes have nothing to do with each other except if their circumstances change. The poor make up the unemployed, abandoned and forgotten in the Whitechapel/East London slum district. In the coming years when Victoria becomes queen, the poor will be the "hot topic" among the upper and middle classes as a way to see which of the two classes would actually be solid enough to take care of the problem on their own. These poor persons are the central characters of all the workhouse stories of Charles Dickens.