Anthropologically, a tribe is a group of people united based on their social organization, belief systems, family relationships, common geneology, and shared language and culture. A more conservative definition includes the caveat that the peoples who live as tribes operate self-sufficiently in their social groups outside of mainstream society and developed as a distinct people many generations ago, before the modern incoporation of nation-states. A more general definition, on the other hand, opens up the meaning to include any groups of people who are united to one another with a collective leader and idea. This can extend beyond ancient peoples to include bands of people that form to collaborate on issues or even behind a music group.
In the United States, the term is probably most familiar in reference to Native American tribes. There are 562 federally recognized Indian tribes, and many others that have not yet achieved federal recognition. 229 of these tribes are located in what is now Alaska, and the others are spread over the United States, some much larger than others. Some tribes have formed distinct, sovereign nations that span multiple states.
These tribes have their own customs, traditions, spiritual views, origin stories, languages, and ways of life developed over thousands of years, long before European settler colonialism.
The more conservative definition of a tribe that specifies the people as living outside mainstream society is not fully appropriate when discussing Native American tribes in what are now the United States. In many cases, tribes have adapted their traditional practices to contemporary times, and many tribal members do not necessarily live in the same place as the majority of their tribe (in the US, often on federally-designated reservations). However, they can still considered a part of the community.