According to rumors the first beta version of Office 15 will likely be released in late January next year. Technology Preview version even likely to be present at CES 2012 in early January. The final version is expected to present at the end of 2012 around December.
The most significant changes from the version of Office 15 is certainly the use of Metro UI which will create the latest version of Office this seems compatible with Windows 8. With the use of Metro UI is almost certain that Office 15 will be present both on Windows 8 version of the desktop PC or Tablet versions.
Use of Metro UI in Office 15 is expected to provide users face a more easy and more focus on the documents they create. In addition the use Metro UI also allows the use of touch, stylus pen and mouse.