A “Persona” is a fictional representation of an actual user and is applied in the early stages of product development or product redesign. Personas are vital to the success of a product because they drive design decisions by taking common user needs and bringing them to the forefront of planning before design has actually started. Personas provide the team with a shared understanding of users in terms of goals and capabilities. Once Personas are defined, going through a task analysis exercise with scenarios will offer an inexpensive way to test and prioritize features throughout the development (see lesson #2 in the blog Responsive Design: Getting started).
Narrow and broad-scope personas achieve different goals. Success depends upon knowing the tradeoffs and structuring your personas’ scope based on what you are trying to achieve. (7 min. to read) https://www.nngroup.com/articles/persona-scope/