1) Personas are archetypes that represent groups of real users who share common attributes and needs. They help product teams focus on the needs of target users rather than their own assumptions. 2) Effective personas are based on research such as interviews with real users or experts. They describe a fictional person through demographic details, goals, skills and a narrative day-in-the-life. 3) Personas prevent self-referential thinking and help ensure features meet the needs of the intended users. They provide a clear and memorable target for teams to design towards.