How does Revit know I am different from anyone else? Your User Name in Revit defines who Revit "thinks" you are. This is found via the Application Menu > Options button > General Tab.
[Edit: For versions before the 2010 release - This is found under Settings menu > Options.]
Why does it matter? In a stand-alone project it doesn't. In Workset projects it matters a lot! Two users with the same user name are not regarded as two people with the same user name. They are regarded as the same person working in two files. We don't want Revit to think this because the first of two "Mike's" to Save to Central wins!!!
A good strategy is to just use the same user name you log into your computer with. IT needs your user name to be unique and so does Revit, "That was Easy". This is the default behavior when Revit is installed. The first person to run Revit after it is installed will have their user name stored. For this reason you need to check it if another person uses your computer from time to time or if an IT person installs your software by working directly on your computer and then tests to see if Revit is working properly by running it while logged on as themselves.
The user name is stored in the Revit.ini file (located in Revit's installation folder) and looks like this Username=MyName. If you delete the name Revit will use the logon user name of the next person to run Revit. If you use Settings menu > Options > Username to set the user name it gets stored in Revit.ini and the cycle begins again.
Just to say it again if it wasn't obvious enough the first time, if you do NOT use the Settings menu > Options > Username to change the user name, and the Revit.ini file setting for username is blank, Revit will always apply the current logon user name to the session of Revit. The Revit.ini file value for username will stay empty/blank. It is only when you enter a value into the dialog within Revit that a value is stored in the .ini file. Unfortunately it is hard to prevent anyone from doing it and it is persistent thereafter. You have to remove the .ini file's value again to get it to work again.
It is important to check your user name routinely. If you use the Local File practices I preach you'll get a confirmation of your user name when Revit displays a warning message alerting you to the fact that the central file has been copied/moved.