Terminology differs from customer to customer and from region to region, but I would break down the users this way.
Front office: end user --> power user --> Superuser --> Key User (Process owner)
Back office: help desk --> Jr. Core user (2nd level support personnel) --> Sr. Core user (3rd level support personnel) --> Consultant.
Front office users execute transactions and carry out the day to day business of the company. Power users are just experienced users that most folks go to if they don't want to call the help desk. Super Users (in the United States) are generally users who get early access to the system during it's initial implementation and are expected to support users out in the field while they do their normal job (it's a power user, but they're more experience due to a formal program during initial implementation). Key users or process owners are generally very experienced personnel who have some decision making authority. If a process isn't working correctly, this person is empowered to make the decision to have it changed. S/he doesn't do the change him/herself, but major changes don't happen without this person's say-so. Key users might be department heads but are often the person who has been in the department the longest and know the history of every system that has every been used to do whatever their task is and can trace the reason for why things were done a certain way.
Back office users support the system itself. They handle all of the support calls when things don't work right and they make changes to the system (configuration or development) when necessary. Help desks,when run well, fall into line as the first level of support and triage problems. Helpless desk folks just act as a switchboard. True help desk folks actually try to solve the problem on their own. If they can't handle it, they forward the issue to a Jr. Core User. (I've never actually heard the term, Core User, in the US, but it fits. Usually, they're called "functionals" or "developers"or "basis", but there are multiple levels. The more senior support folks set direction and do the heavy lifting when things go wrong. Consultants are usually only brought in to do implementations or upgrades and then leave once the system is working. In a perfect world, they're the most knowledgeable in the chain. In the real world, they come with different experience levels as well and canbe more or less helpful depending on that experience. They're not really at the end of the chain as much as they are a temporary parallel chain to the support folks.
Hope this helps!
Best regards,
--Tom
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