Power Pivot introduces a new formula language called Data Analysis eXpressions or DAX. When you see a Power Pivot demo, the 2 million rows and the joining tables look impressive. But it turns out that DAX is the really jewel in Power Pivot.
DAX is used in two places. DAX is used to add new columns in the Power Pivot grid. When used in this way, it is 99% similar to the functions that you know and love in regular Excel. The real power in DAX is when you add new calculated fields to the resulting pivot table.
While I am going to cover some essential DAX examples here, the my friend Rob Collie from PowerPivotPro.com has written the essential title on understanding DAX. His book is DAX Formulas for Power Pivot.