Email, Exchange, OWA, Outlook Web App Light versus Premium (regular)
There are differences in the available features between OWA "Premium" and OWA Light. There is a check box on the log in page for enabling Outlook Web App Light Version.
There are differences between the Outlook Web App and Outlook Web App Light interfaces. Some features are not available in the light version.
However, some web browsers may display the light version regardless of what option is selected on log in. For example, Chrome on Linux is a browser which forces the light version. Firefox on Linux does not force the light version.
For a comprehensive list of the differences between OWA versions, see: http://blogs.technet.com/b/ilvancri/archive/2010/04/07/exchange-2010-owa-versus-owa-light.aspx