When it comes to internet marketing, a niche loosely refers to a segment of a larger market that can be defined by its own unique needs. You'r writing, adverts and all will be structured to carter to this particular segment.

When it comes to affiliate marketing however, you will be required to create a blog with frequent updates of some form or other. Suffice this to mean that you’ll need to write about a topic and find an audience who are interested in either reading about it or are in need of the proffered solution. 

Simultaneously, your chosen niche will to a very large extent define the type of product you sell. If you write about relationships, then you’ll need to sell relationship-related products. If you write about losing weight, then you’ll need to sell something to do with fitness and all that. You may find that products/services tend to gravitate more toward the high end or the low end depending on your chosen niche.