DAX what is it?

Data Analysis Expressions, or DAX, is the native formula and query language for Microsoft PowerPivot and SQL Server Analysis Services Tabular models. DAX includes some of the functions that are used in Excel formulas, and additional functions that are designed to work with relational data and perform dynamic aggregation. It is, in part, an evolution of the Multidimensional Expression language developed by Microsoft for Analysis Services multidimensional models combined with Excel formula functions. It is designed to be simple and easy to learn, yet to expose the power and flexibility of PowerPivot and SSAS tabular models.