PowerPoint Web App has been designed as a companion to PowerPoint desktop application. Web app’s focused on giving you access to your presentations wherever quickly & easily. PowerPoint Web App’s Reading View is designed for quickly reading presentations that’ve been published on web. Here we’d introduce you to Edit View – a place where you can changes your presentation on web. This view is designed to parallel desktop app’s Normal View. On web, you’ll find familiar ribbon, thumbnail pane, and notes pane. Take a look at desktop app and web app side-by-side:

PowerPoint 2010 Normal View             PowerPoint 2010 Web App Edit View
 

You can go directly into Edit View (click Edit on Skydrive or SharePoint) or switch over from Reading View. If you are in Reading View, the toolbar across the top of the view has an entry point, shown below (1st pic). Notice that when Edit View opens, you’re still on same slide you were just reading! If you want to see how your slides will look animated, you can switch back into Reading View or check out your progress in Slide Show from View Tab.

PowerPoint Web App Reading View Toolbar                              PowerPoint Web App Edit View, displaying View Tab
 

More info: PowerPoint blog