Niche marketing is when a firm specialises in serving one market segment with unique demand characteristics.
For example: The Bobby Grace company specialise in one product: the putter. They only make putters and have a very small product offering of accessories beyond that. Their putters are marketed as highly advanced technologically because all of the company's attention is dedicated to this one club, the putter.
Differentiated marketing: firms that serve multiple markets segment each with a unique product offering.
For example: The Toyota company has different products such as Lexus for the luxury segment and Scion of the younger drivers.