Personas
Personas are fictional characters, which you create based upon your research in order to represent the different user types that might use your service, product, site, or brand in a similar way. Creating personas will help you to understand your users’ needs, experiences, behaviours and goals.
Articles

A Closer Look At Personas: What They Are And How They Work
Personas help to keep a designer honest and to become mindful of when they are truly designing for others and when they are just designing for themselves. If you are going to design for someone unlike yourself, then do your users a solid and use a persona.

Creating Personas - A guide, not a template
Personas are all about building empathy amongst your team. Great software gets made when the people who make it care about the people who use it. That means during every meeting, when making any decision, in every design and with every line of code, you should first be thinking about your users.

Data-Driven Personas at Salesforce - Understanding Our Users in a Whole New Way
They’ve landed on data-driven personas as an efficient, research-led method for better understanding their users. User personas based on data provide their designers, product managers, doc writers, engineers, and executives with a common shorthand for their users that’s both easy to use and is based in current data about their user base.