DAX is the query and calculation language of the Power BI Data Model. DAX can be used to create three types of object in Power BI Desktop: calculated columns, which add new columns containing derived values to an existing table; measures, which control how data aggregates up in reports; and calculated tables, which are tables derived from other tables loaded into the Data Model. One use for calculated tables is to create a Date table in your dataset, which is essential if need to do any date-based calculations - although there are several different other ways to create Date tables, for example using M in the Query Editor.
When writing DAX you should follow the best practice of writing column references in the format ‘Table Name’[Column Name], and writing measure references in the format [Measure Name]. This helps you distinguish between columns and measures when you are reading DAX code.
There is a lot of overlap between what you can achieve with calculated columns and calculated tables and what you can achieve in the Query Editor, and indeed what you may be able to do in your data source, for example with SQL views. Generally speaking, the earlier in the data import process you can perform any data transformation and manipulation the better.
