Leila Etaati provides an explanation:
In Fabric, there are tools for different personas of the users to work with. For example, for a citizen data analyst, Dataflows and Power BI Datasets are the tools with which the analyst can build the data model. For Data Engineers and Scientists, one of the tools is Notebook.
The Notebook is a place to write and run codes in languages such as; PySpark (Python), Spark (Scala), Spark SQL, and SparkR (R). These languages are usually familiar languages for data engineers and data scientists. The Notebook provides an editor to write code in these languages, run it in the same place, and see the results. Consider this as the coding tool for the data engineer and scientist.
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