This question causes much confusion. It's a difficult subject in detail, but on a basic level it need not be.
Patricians were originally, as you say, the oldest families in Rome, selected by Romulus (supposedly) to form the original senate. By the late republic, these original families had declined massively in numbers and in political power - descendents of plebian families outnumbered them in the senate and could as rich, or richer than they. Effectively, by this point the patrician distinction was no more than a family honorific, connoting no special political authority.
Emperors from Augustus onwards tried to rejuvenate the patrician class by giving patrician status to deserving plebian senators (Agricola is a good example - originally of a provincial family from Gaul). But again this status was merely an honour to the family name, not a political position. This had the effect, over the centuries, of further reducing the ancestral prestige of the patriciate, until by the third century the term had lost all real meaning. It was revived again under Constantine, and in the late empire came to have a different meaning.
The equestrian order was originally formed as a military (cavalry) caste by selected members of both plebian and patrician families, based on wealth. Again, by the late republic this had changed, and the equestrians had become the lower aristocracy. Sons of senatorial families (ie families whose paterfamilias was a senator) would be entered into the equestrian order until such time as they were elected Quaestor and began their own senatorial career (whereupon they had to give up their equestrian status).
Generally speaking, the old distinction between patrician and plebian families had become almost meaningless by the end of the republic - the senatorial/equestrian divide within the aristocracy was much more valid. Confusion can arise when the word 'plebs' is used as a synonym for 'the masses' (Roman writers themselves did this - Juvenal for one, I think). The meaning in this case is different to 'plebian' as a familial distinction. Similarly, a lot of the sloppier modern writers, particularly for film and TV, use 'patrician' as a synonym for the wealthy senatorial order - HBO's 'Rome' was particularly guilty in this respect!
Beyond this very basic summary, things get knottier and more complex, but I'll leave it to others to expand (or correct!) as they see fit
- Nathan
