A belief-tribe is any group of people who follow a common belief system. The group need not be formally affiliated in any way, and typically are not an actual tribe in the sense of a pre-state society.
Members of a belief-tribe will often signal their membership by the use of certain unusual words or phrasings, following certain customs, and (of course) promoting the tribe's beliefs in public venues whenever given the opportunity.
There are several major types of belief-tribe common in today's global meta-society:
The ideal scientific belief-tribe can be summarized as "the tribe that believes in questioning what the tribe believes in", since science is based on the idea that any belief must withstand skeptical inquiry if it is to continue.
In practice, many scientific tribe members pay insufficient attention to the understanding of their beliefs and, as is human nature, revert to easier heuristics such as accepting the word of scientific or supposedly-scientific authorities (e.g. people with science degrees employed to advocate a company's political position, politicians who use the fact of their past scientific training to boost their apparent credibility, actors dressed as doctors in advertisements, actual scientists speaking outside their fields of expertise, engineers and technicians speaking above their level of understanding) without question or believing anything that is suitably decorated in science-y trappings (e.g. pseudoscience, fake journals).
To some extent, this is understandable, since it is quite impossible for any individual to retain enough scientific knowledge to evaluate every possible claim at an expert level. While there are methodologies for laypeople to use in evaluating the credibility of scientific claims, these are generally neglected in public education and there are few if any definitive references available.
A scriptural belief-tribe is one which holds that certain writings represent infallible truth received from some higher being or plane of existence. These writings are generally called scripture, and the sets of beliefs subscribed to by these groups are called religions or faiths.
There is ongoing conflict globally between such groups with differing sets of beliefs (due to different scriptures being followed) as well as between (on the one hand) scripture-based belief and (on the other) evidence-based belief; at its core, this conflict is essentially between authoritarianism and individualism (or at least non-authoritarianism); see reason vs. religion.
Another large area in which belief-tribes form is in the political sphere. These belief-tribes seem to form in response to one or more perceived threats (whether real or not), and the beliefs – often having nothing at all to do with any evidence or rational goal – usually can be traced, often via quite tattered chains of association, to those threats.
The "American conservative" belief-tribe, for example, coalesced around fears of loss of white, male, Protestant privilege. The 1950s "cold war" against communism was, in this regard, an expression of fear that alternatives to the established order of capitalism might become popular (as indeed they did).