Power BI dataflow is Power Query Online. It can do what you do with Power Query Editor in Power BI Desktop. It is used to connect to data sources, transform and filter data, combine multiple queries, etc. It deals with the ETL process of data. It doesn't include data model (relationships between tables, measures, calculated columns, calculated tables). However a Power BI dataset has data model in it.
You can refer to this article to learn more differences between them: Power BI Datamart Vs. Dataflow Vs. Dataset - RADACAD . Power BI Datamart is newly introduced in this year. You may have an interest in it.
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