A domain user allows you to log into the Plesk control panel using the domain name as the user name with the password that you assign to that domain user. A domain user is only able to access information about their domain, and cannot change any of the limit settings for their account.
A web user can have a user area associated with the account. A web user can upload files to that site and can have a website at http://mydomain.com/~myusername, where "mydomain.com" is the actual domain name and "myusername" is the actual web user for that domain.
