Short answers, pulled from the story.
Romulus created the three original Roman tribes known as Ramnes, Tities, and Luceres shortly after founding Rome. These tribes divided into ten curiae each to serve as voting units for the comitia curiata.
King Servius Tullius restructured tribal divisions when he became the sixth king of Rome during the sixth century BC. He abolished the Romulean tribes while preserving their curiae and established four urban tribes plus seventeen rural tribes.
The final two tribes Quirina and Velina were established in 241 BC bringing the total number of Roman tribes to thirty-five. No new tribes were created after this date despite subsequent territorial annexations.
The comitia tributa functioned as Rome's most democratic assembly where all citizens received equal votes regardless of wealth or status. This body elected lower magistrates including tribunes of the plebs and military tribunes while passing resolutions on domestic and foreign matters.
Membership in a tribe proved Roman citizenship and formed the basis for army levies throughout Roman history. Enrollment served as prima facie proof of citizenship rights until at least the third century AD despite political changes.