The best way to think about "a niche" is a tribe that gathers around a specific interest. Tribes make superior target groups, because they describe how we relate to other people, and also how information spreads. If your target group is a tribe, they share lots in common that you can use to your advantage:
1) They gather in specific places. Benefit: You can find and target them efficiently
2) They speak a common language. Benefit: You can communicate natively
3) They listen to tribal influencers. Benefit: You can partner with tribal influencers
4) They attend recurring events. Benefit: You can create, sponsor, or attend these events
5) They enjoy sharing similar stories. Benefit: You can create content that they will love
6) They are already courted by other brands. Benefit: You can partner with these brands
7) They share common beliefs, values and world view. Benefit: You can be extremely relevant
8) They are share a passion or interest. Benefit: You can connect through emotions

I recommend further reading:
1) How to research tribes, using The Tribal Map
http://tribaling.com/blog/2013/06/17/find-your-way-across-the-tribal-web-with-a-map/
2) How to do Tribal Marketing in social media
http://tribaling.com/blog/2013/07/17/tribal-marketing-in-social-media/
3) Understanding the 7Cs of Tribal Influence
http://tribaling.com/blog/2013/08/27/the-7cs-of-tribal-influence/