A tribe is a traditional social group of people. Most tribes have existed much longer than existing states and countries.
Tribes follow rules and traditions that aren't connected to modern-day governments or mainstream societies. Indigenous people of North America lived (and many continue to live) in tribes, although the word itself is controversial — many of their members prefer the terms nation or ethnic group instead. Sometimes tribe is used to simply mean "family" or "clan," or even a large group: "A tribe of college students roamed the streets."