Autodesk Revit launched without a valid license (after the Trial period) works like a "viewer", a free tool for viewing a printing of projects, without any editing.
If you need to run Revit in the Viewer mode even on a computer containing an active license (e.g. to save a floating network license), you can invoke this mode by a runstring parameter. Run Revit with the following command:
"C:\Program Files\Autodesk\Revit Architecture 2012\Program\Revit.exe" /viewer
Revit will be launched in a special mode, it will not consume any license and its window title will contain the text "VIEWER MODE".
You can do the same with the predefined Start menu item: Start > Programs > Autodesk > Revit Architecture 2012 - run "Revit Architecture Viewer 2012".
You can also use the Viewer mode as a Demo mode for the freely downloadable Trial version of Revit (= after the Trial period).