What are Personas?

Often, personas are described as archetypes of your target groups. But what does this mean?
The target group is more or less known. For instance it is college graduates or women between 30-40.
Target groups have their value, but fall short in describing the needs, triggers, values, and requirements of potential individual customers. A persona gives the target group a face, history, and biography. Personas will indicate whether or not the customer would buy and why.
The difference between target groups and personas
A simple way to understand the differences between target groups and personas is to define a target group and its results. For instance:
British
There are quite a few Brits around. 66.47M in 2018. However, this does not define an effective target group.
Narrow it down further by targeting British people, male, and divorced. No exact statistics could be found, but still no exact target group.
Narrow the targets a bit further: Male Brits divorced one or more times whom live in a castle.
This makes for a finite homogeneous target group, doesn`t it?
Resulting in two prominent members of the target group:
Ozzy Osbourne - Prince of Darkness
and
Prince Charles - Prince of Wales and heir apparent to the British throne.
Would you address both gentlemen in the same manner?
Here is where personas chime in:
Personas put a face to the abstract descriptions provided by the target groups and help address the people behind the numbers in a way that engages them with your content, making them more likely to buy or sign up. This implicit customer centricity shaped the idea of personas.
Kathy
The origin of personas can be traced back to 1983 and Alan Cooper.
In an article of the same title, he described how he grew the idea behind personas. Back then, computers were slow. At least once a day, Cooper, working as a software developer, needed to compile a large project management tool. Thus, during that wait time, he would walk and engage in a fictional conversation with a non-fictional user he named Kathy.
He discussed the problems in usability that Kathy faced while using the project management tool with her. As a result, he used these insights to develop further the software, which became a significant success.
Chuck, Cynthia, and Rob
After 1995, Cooper, then a consultant, was tasked with helping one of the first Business Intelligence tool vendors understand their customers. He started interviewing potential clients of the BI tool and figured out that most fell into three distinct groups. Goals, tasks, and skill level. Hence, three personas were created: Chuck, Cynthia, and Rob.
Cooper called personas his secret interaction design weapon and started utilizing it in all of his projects. Witnessing developers talk about personas and what they would do, or if they would understand a feature. Learnings during this process led Cooper to publish in 1998 "The Inmates Are Running the Asylum", which is now the standard.
Subsequently, personas became a wild success across industries and organizations. They are being utilized in everything from marketing to development, whenever a customer focus is needed.