A good intro to using personas in a piece called Setting the Stage for Building Usable Information Sites:

Project stakeholders, or "higher-ups," may initially greet the new idea of personas with some resistance. Personas frequently fall under fire because they are misconceived as being no different from traditional market segmentation tools.

Although both planning tools can be effectively used together with some positive results, market segmentation and personas are quite different. Market segmentation is a quantitative forecasting tool that provides a breakdown of a consumer market and predicts someone's willingness to buy. Market segmentation derives findings from large samples with averaged data about demographics, behaviors, and attitudes.

By comparison, personas are a qualitative decision-making tool. A small set of one-on-one interviews serves as descriptive fodder for determining a set of specific characters that represent the same goals of many likely users. Personas, in turn, enhance team decisions about the site's design, especially what features need to be included and how the site will be used.