In England, it was strictly tied to titles. Being made the duke, the earl, the baron made you noble. Your children, even your heir, were commoners until one of them inherited, and only the heir became noble. Though titles were granted, and occasionally you could even resurrect a peerage out of abeyance.
In France, any noble's child was noble. It was even conferred through the female line. Since most titles were not thus tied to the legal prerogatives, they were rather more loosey-goosey, and indeed, some royal officials dismissed the notion of investigated fraudulent titles when they weren't one of the handful that actually got rights.. After all, what it got them was a different form of address from their social inferiors. There were still ways in. Money, for instance, or official posts. Not that that meant you were well-off, A large percentage of noble household in ancien regime France had no servants at all.
In Venice, you were noble if your family was written in the Golden Book. You would be enrolled at the age of eighteen. No getting out of it -- with all its duties, no matter how poor you were -- and virtually no getting into it. To be sure, being forced to be noble has long been a widespread problem, especially when it means you can't work, or can't work without losing your noble prerogatives that are all that's protecting you in some other situation.
Of course, then you start to get the moralized meaning: noble is not someone of a given rank, but someone who behaves fittingly for it, and five seconds after that one gets "noble is as noble does" and a wedge between the rank and the moral meaning. Still, it gives a glimpse in, because what virtue is enthroned tells you something about the noble class. "Frank" for instance, or "gentle" or "noble." Some, of course, need some digging because of semantic drift, such as "cavalier" as an adjective, which no longer carries the same connotations of "nonchalance." Or "ingenuous," which still sometimes means "honest" but no longer carries its old connotations of modesty and ability to be disinterested. (C. S. Lewis's Studies in Words traces some interesting permutations of status words.)