In his book Tribes, Seth Godin describes the processes by which fans gather around organisations that are important to them. He relates how they follow leaders, visionaries (or heretics as he dubs them) and that this quality is what brings the tribe close together.
A tribe is a movement of fans that emerges from within a niche with the energy to re-order everything. Tribe members have a cause, a leader, a marketing story to tell and a passion to tell it. They do your marketing for you, attracting more and more fans through their enthusiasm.
Tribes can evolve from either growing or dying niches. They sometimes flame out, complement or eat their original niche depending on circumstances. Some tribes massively outgrow the niche from which they emerge. Others remain a small part of a larger niche. Sometimes tribes that come from niches are reincarnations of older ideas in new forms. At other times they are sub-divisions of an existing niche formed for ideological reasons. Ideological tribes are usually the hardest to grow.