Clentela (the patron-client relationship) was, at its heart, a system of hierarchical relationships that structured the social networks of those in Rome. It was non-contractual and in most respects informal, although the law did recognize the existence of the relationship and so patrons and clients could not oppose each other in law suits, for example. Beyond that, there are few generalization that can really be made: they can range from the relationship between a politician and his political "crowd" that followed him around to increase his profile to the relationship between cities and even provinces and well liked governors. In the Late Republic, for example, one of the few ways provincials could seek redress was to put themselves under the political protection of a prominent Roman senator. Clientela could also be between an established leader and an up-and-coming politician, for example, I find it somewhat likely that Sulla had begun his career as Marius' client or something similar. The most important and notable patron-client relationship was between the Emperor and the Empire, particularly the city of Rome.

The common way I personally encounter clientela is through the broader relationships of amicitia, literally "friendship", that played an important role in structuring economic relationships. For example, it was possible for a client to use his relationship with his patron to communicate his trustworthiness, eg, if the patron is well known than his clients can use him as a "reference", so to speak. Likewise, a patron can use their clients to conduct complicated business dealings in which they didn't want to assume full risk. These sorts of semi-formalized social networks allowed for a greater degree of complexity in trade and commerce than would have otherwise been possible in a world without modern communication or political centralization.

The best recent book on this is probably Koenraad Verboven's Economy of Friends, which deals heavily with legal aspects.

There are some scholars re-evaluating this relationship. People used to think clientele were hereditary and obligatory. I have heard Morstein-Marx talk about clientele and he is leaning towards thinking clientele was much more fluid than historians have usually thought - because of things like canvassing for office and mutinies in the army. If I remember what he said correctly, he thinks a patron/client relationship was very much a two way street, in which the client can walk if the patron doesn't provide enough services.

In reference to your question about education, I think we are re-evaluating our notions of literacy. Many Romans would be able to read something - street names, a menu, etc. Maybe not Virgil, and maybe not Greek, but for daily activities they probably would have some capacity to function. Also, Rome was an oral culture so if you don't read you just need a friend that does. At Pompeii there are menus on the walls - they must have had practical use somehow.

The patron/client system is one of the central cruxes of Roman society throughout its existence but one whose power diminished somewhat during the empire. It started when Rome was still small and essentially operated as a gang would today. You have the wealthy center as the patron and the clients as his posse/entourage. Basically you were the patron's man and you did as he bid, in return you got food, work, and general privileges. The wealthier and more useful you are the more you get in return. They are somewhat important for votes in elections when the electorate broadens a bit with the reforms of the Gracchae brothers but mostly they are important for the prestige they bring to you. Everyday the patrons bring their entourage and parade it around the forum. For Senators in particular a large number of patrons meant very real power.

Basically as a client an everyday citizen would get protection from his patron and gifts in return for leal service.

I can say that Marcus Mediocris would have had a web of connections in his daily life if he had a patron and possibly clients under him. The system was a hierarchy where people took on clients for prestige and clients took on patrons for assistance or the social connection. If Marcus had been challenged legally, he could use his patron as a defender if his patron is a good orator, or his patron would assist him in finding one. His patron would also include him in a kind of family connection and in return, Marcus would be obligated to assist his patron if he needed it, and Marcus could not testify against his patron. He would also help his patron build his reputation if Marcus had the means to take on a client himself.

The twelve tables have some bits of useful information on the legal aspects of this relationship.