Office Web Apps is essentially your Microsoft Office in the cloud. "Enterprise customers with on-premise SharePoint installation, OWApps requires SharePoint Foundation 2010. On the other hand, OWApps does require volume licensing. Web Apps can present Word, Excel, and PowerPoint files on many devices. Supported mobile viewers for Web Apps on SharePoint include Internet Explorer on Windows Mobile 5/6/6.1/6.5; Safari 4 on iPhone 3G and 3GS; BlackBerry 4.x and later; Nokia S60; NetFront 3.4, 3.5, and later; Opera Mobile 8.65 and later; and Openwave 6.2, 7.0 and later. To roll out services in an enterprise environment, TechNet has documented specifics including planning and deploying OWApps."
"For consumers, OWApps are part of Windows Live offerings. A user with Windows Live ID can user Web Apps to create, upload Office documents which're stored in SkyDrive. Supported mobile viewers for Web Apps on Windows Live include Safari 4 on iPhone 3G and 3GS, and IE7 on upcoming Windows Phone 7. Viewing Excel files via a mobile browser is currently only available with Office Web Apps on SharePoint 2010."