Janet is a sentient database for the Good Place. She was (presumably) created by the 'Eternal Beings' e.g. Michael. Her purpose is to "Make your (after)life easier!" Janet is able to instantly appear to anyone's side when they call her name.
Janet is used primarily as a personal assistant by the residents of the Good Place. Most people summon her by saying 'Hey Janet'. She is able to retrieve/create any object requested for instantly. Janet also doubles as an accessible database, as she reportedly has all the knowledge of the universe. The only forbidden information she retains is about the Bad Place. Janet is only permitted to play a brief audio clip of what is happening there at the moment of when you ask.
Janet by default, has an extremely cheery personality type. She is always courteous and non judgemental by design. It is impossible to insult her as she cannot feel sad. The only exceptions to these are when she is asked to alter her personality or when she is murdered/reset.
Janet is not human or an Eternal Being, she is technically an Artificial Intelligence. In the episode ' Category 55 Doomsday Emergency' Michael scolds Tahani that the manual for the town is only to be viewed by Architects and Janets. This implies there are multiples of Janet at any given time potentially one per neighborhood. Janet states that there are 25 versions of Janet. This means she is upgraded similar to a computer program. She also refers to herself in the third person when discussing herself as if she is an object. In Chidi's Choice, Janet gets married to Jason/Jianyu.