"Tribes" is the term Seth uses to describe groups of people who form around an idea and a leader. It has huge implications for marketers, since it changes the approach about marketing your product or service or idea. Instead of making sure Everyone knows about your idea, Seth asserts it's more important that you reach out to those people who are passionate about your idea, give them the platform to let your idea spread and lead them along the journey.
It's tougher for me to explain the concept than it is to share examples. I figure Seth anticipated that, since he organized his own tribe (including me) to build an ebook with tribal examples. There are nearly two hundred cases in the book, including a short one about my favorite fantasy football website, footballguys.com. Go check it out, or download it here.
While you're at it, make sure to sign up for updates on Seth's site. It's my favorite blog in the world, daily inspiration, observation and marketing leadership. Seth knows what's going on.