Basically said, everything is a niche. There are big niches and small niches, take Politics for example. There is world politics, US politics, Californian politics, Arnold Schwarzenegger politics, New York politics, Manhattan politics and so on. It is generally easier to compete in a smaller niche because there is less competition, if any. It also means less traffic if you compare spots on search engines but most niches provide enough visitors to create a successful website that adds great income.