Clientela was a Roman law, or social convention that linked Plebeians with the legal, social, and sometimes economic protection of Patrician families. This was not only customary, but necessary, as Plebeians on their own had limited legal rights and protections under the law, and could not legally enter into contracts.
Anyone - usually a Pleb - who required protection or assistance of a more powerful family could petition them to become a cliens, and accept the head of the more powerful family as their patronus (this is clearly from where the modern terms patron and client are derived). Theoretically, such a relationship could be temporary, once the "favor" or obligatio was discharged - if it were a matter of money, or political support, for example. In practice, such relationships were very long term, often multi-generational as the obligations of clientes and patronii were legally and customarily passed down from father to son.
The cliens and the patronus had mutual obligations to one another.
Clientes were required to attend their patron's "levy" on a regular basis to see if there was any service that they could render them. Cliens were expected to contribute towards the dowry of their patron's marriageable daughters. Clientes could not be compelled to testify against their patron in a court of law. Traditionally, clientes accompanied their patrons to the Forum, and in times of war, as vassals. Clientes were also responsible for helping their patronus defray losses from legal actions, and the ransoming of captured family members. There were other restrictions, both social and legal. Clietes took on the gens name of their patron, something else that was hereditary. While a family existed as clientes they could not be considered a separate family, nor could they form their own gens. See Roman Naming Conventions.
Patronii also had obligations to their clientes. The patron was the legal adviser of the cliens; he was the client's guardian and protector, as he was the guardian and protector of his own children; he maintained the client's suit when he was wronged, and defended him when another complained of being wronged by him: in a word, the patron was the guardian of the client's interest, both private and public. Like the clientes, the patronii were immune from being compelled to testify against their clientes in a court of law.
The acquisition of clientes by the patronus was also a matter of social distinction and status. Simply put, if you had a lot of rich and powerful people who owed you obligations (obligatio), then you must be even more rich and powerful.
By the time of the end of the Roman Republic, extremely powerful Patricians had entire foreign city states and nations as clientes. In such cases, the Roman Senate often would settle disputes between foreign nations within its "sphere of influence" by referring the matter to be resolved to the patronii of the respective disputing client nations, and abide by the results.