On a SharePoint 2010 site where Microsoft Office Web Apps have been installed and configured, Office Web Apps give you browser-based viewing and editing of Office documents from anywhere you have a connection to your organisation's SharePoint site.
If you have Microsoft Office 2010, you can save Word, Excel, PowerPoint, and OneNote documents directly from your Office program to SharePoint. Even if you don't have Office 2010 you can store documents in a SharePoint library and start using Office Web Apps right away.
When you install Office Web Apps, the Office Web Apps Services are added to the list of SharePoint Services and the Office Web Apps Feature is added to the available SharePoint Features.
When you click on a document stored in a SharePoint document library, the document opens directly in the browser. The document appears in the browser similar to how it appears in the Office client application. The Web app also provides many of the same editing features as an Office client application.
What will happen if your are away from your desk and you need data from your presentation or spread sheet you worked on earlier but it's only saved on your personnel computer?
Office Web Apps enables you to use your cell phone to read Microsoft PowerPoint, Microsoft Word, and Microsoft Excel files in organisations that have installed and configured Microsoft Office Web Apps on their SharePoint 2010 site. You can catch up on a report or review an important presentation while you're on the go even if your phone doesn't have Microsoft Office Mobile 2010 applications.