In brief:
The purpose of market segmentation is to identify different groups of customers (or potential customers) within a market so that it is possible to target particular products, services or marketing messages. Once identified, these groups are referred to as segments.
Segmentation is a powerful research output, but it does not delve into personal stories, goals or motivations. It does not provide rich insight into user experiences, journeys and pain-points along the way. Unlike personas it is not a design tool.
Where segmentation seeks to identify and measure the size of different groups at a high-level, personas are devised to provide a rich understanding of the user context, needs, motivations, behaviour and the associated design challenges / opportunities.
http://www.foolproof.co.uk/thinking/why-market-segmentation-is-not-the-same-as-personas/