As per this Microsoft press release.
The non-concurrent use agreement in the Microsoft Office 2008 EULA has been removed from the 2011 version (boo!). This means that if a single user has two computers (desktop and laptop or home and office) then the "Multi-Pack" version is required (see below).
In addition, there are no more 'upgrade' vs. 'retail' editions (awesome, less confusion). Most of our clients will need one of the following items:
• Microsoft Office 2011 Home and Business Single User ($199.99)
• Microsoft Office 2011 Home and Business Multi-Pack ($279.99)