Niche marketing refers to specialized products or services that answer to the need of a relatively limited target ( compared to mass marketing). Moreover, consumers of niche produits relate to each other (by their hobbies, financial means, location… Or else)
Mass marketing is more focused on the consumer need to fulfill than the consumer type to target. The target is often wider and less precise.
To make it very simple : a pen is a mass marketing product, while a pen especially designed for left-handed belongs to niche marketing.