How are personas and market segments different?

Personas and market segments are highly similar in the sense that they both are based on real data about the customers. However, the main difference is that personas individualize that data in a representation that has a name, face, and human attributes. In conclusion:

Personas = human like representations of customer groups
Market segments = nameless, faceless descriptions of customer groups

Jansen, B. J., Salminen, J., and Jung, S.G. (2020) Data-Driven Personas for Enhanced User Understanding: Combining Empathy with Rationality for Better Insights to AnalyticsData and Information Management. 4(1), 1-17.