Microsoft launched a consumer preview of the new Microsoft Office suite this week. Traditionally, the core Microsoft Office elements have been Word, PowerPoint, and Excel, but OneNote has developed into a fourth pillar of the Microsoft productivity suite. In fact, Microsoft has put OneNote front and center as the poster child of the new mobile, touch-friendly Office.

First–in case you’re coming late to the party–let’s try to define what OneNote is. We all know Word is for word processing, Excel is a spreadsheet program, and PowerPoint makes slideshow presentations. OneNote, however, is a bit more difficult to label so succinctly.