Personas are fictitious representatives of real people.
Alan Cooper's company, Cooper, uses personas to abstractly represent users for which their software is being designed. The personas contain detailed information about their social and professional lives.
Small teams of interaction designers analyse these personas, re-enacting different scenarios in order to discover their goals. Results of the analysis are incorporated into future software development.
Although the software should fulfill many different users needs, it should be designed for one persona, since this persona represents the largest group of users that will be using the software.
The users that are represented by the chosen persona are the most likely to buy the product since the software satisfies their needs.