A persona is a measurement model, which started to take off in the 90s, as a way of marketers to be able to tangible depict a person, not a real person but rather “synthesized from observations of many people”.
Each persona represents a significant portion of people in the real world and enables the designer to focus on a manageable and memorable cast of characters,
Personas for me today, when working on mobile projects, allow us to understand the range of customers that would use our completed app, and in an iterative agile methodology, identifying the right personas (especially in the early stages) allows for concept validation and re-validation, the ability to test theories and ensure it meets the target market.
What are personas?
Conceptually, a persona is a one-page document that summarises research trends and patterns of a particular targeted market segment, as a person, with a fundamental emphasis on understanding the person.
Take a look at a great article by Smashing magazine which goes into great detail on Personas: A Closer Look At Personas: What They Are And How They Work (Part 1) | Smashing Magazine