Middleground and Jimbo,
Web User is exactly what Jimbo said, and you will always recognise a Web User site, by its URL. For example:
http://www.sassinc.net/~Heights
The Web User site always starts with a tilde (~) and never ends with an internet extension such as .com.
A
Domain User is an entirely different animal and really only applies to resellers. If you are a reseller, and you just sold space to Joe Red Wing, you want him to be able to access his own Plesk control panel, but be restricted to only his site, etc. Plus, if you have taken the time to install your own logo in Plesk, you want him to see that. So, when you setup Joe Red Wing's account, you also click on Domain User and enter a password for him - say
StanleyCupChampions. Now when Joe Red Wing logs into Plesk, he simply enters the name of his domain as a user ID
(wearenumberone.com) and his password
(StanleyCupChampions) and wham - he enters his own Plesk space!
Hope this helps!
- Mike