Someone may correct me, but I don't think "concurrent users" and "simultaneous users" are tightly-defined terms. I think they both have no more than their plain English meanings, which I would say are pretty much identical.
If told a Web site had 150 concurrent (or simultaneous) users, I would not think that it meant that 150 requests were being processed all at once. I would think that it meant that 150 users were logged-on and interacting with pages on that Web site. But many of them would be reading the page, thinking, or typing responses, and thus wouldn't be loading the server.
However, as these terms do not have tight definitions, another person might interpret them differently. They might really mean that there were typically 150 requests being processed simultaneously. Because of the large proportion of idle time on most Web sites, that would translate to thousands of users logged-on to the site, at that time.
Betty Rubble? Well, I would go with Betty... but I'd be thinking of Wilma.
I agree with peter that it is open to interpretation.
Probably at least important as the concept of concurrent/simultaneous users is the concept of concurrent/simultaneous requests. I track that in jamon and came up with yet another term for this called 'active requests'.
Also most sites have far more registered users than they have concurrent users. But more concurrent users than concurrent requests. Using JAMon we tracked this information on apps where I work and one application had 500 registered users, but typically only 1 concurrent request!
I have also seen one user logged in and 10 concurrent requests as this one user impatiently mashed on the refresh button.