The Office Web Apps, which provide web-based versions of Word, Excel, PowerPoint, and OneNote, no longer require users to sign-in with a Microsoft account to edit shared documents. Anyone with the link can edit the document and save anonymously.
The documents are stored in SkyDrive. You can either share documents from the SkyDrive web interface or, by using the "Get a Sharing Link feature" in Office 2013. Now with the anonymously documents editing, SkyDrive has become a powerful collaboration tool.
Source: SkyDrive blog