If you don't have Microsoft Silverlight 2 or later installed, Word Web App and PowerPoint Web App display a bar with a link to install Silverlight. Silverlight is not required to view documents and presentations in Office Web Apps, but if you have the latest version of Silverlight installed, your experience of the apps is improved.
In Word Web App documents render more quickly and with greater visual fidelity at high zoom levels. In PowerPoint Web App slides are better scaled to the size of the browser window and some animation effects are smoother.
This video provides a side-by-side comparison of the Office Web Apps experience with and without Microsoft Silverlight 3 installed.
Note: This video has no audio content.