nouveau riche is a term, usually derogatory, to describe persons who acquire wealth within their generation, and spend it conspicuously. The implication is that, being of lower- or middle-class origin, these individuals lack the taste to properly use wealth. Hence, this class of people is sometimes ill-regarded by old money as culturally inferior, comparatively lacking in pedigree and subtlety.
The benchmark of the "nouveau riche" is their acquiring posessions which are touted to them as being the sort of things that rich people would possess. "Old money" in traditional European societies have inherited a large house filled with well-built furniture acquired over the centuries. Their cars are not necessarily the most expensive and capable, but have a bit of restraint, or else they disguise their social position in cars of the middle rank. During the Great Depression, Chryslers, for example, were sold to such people for this reason. They fit within a social milieu in which everyone knows everyone else and has for generations. The "nouveau riche" in essence try to crash this party by buying everything they need to show that they have money not merely within one generation but within a few years.
A group of conservative Brooks Brothers/Polo cladded teenagers, rolled their eyes, and thought, "nouveau riche."
One could argue that the only validity of the derogatory moniker is for those who have more money than class - but then that could be said of the more traditional rich.