I've run a vB board since vB has been around (a long time). I'm happy to help any way I can.
The member/senior membor thing is driven by post count and is called the "User Rank". The software can be configured with many levels. For example on a Martial Arts board you could make the members like this:
0: White belt
50: Yellow belt
100: Orange belt
200: Green belt
300: Blue belt
etc.
2000: Black belt
It's completely configurable.
Similarly, a Moderator is someone that can moderate posts in a forum. A Super Moderator is someone that can moderate posts in all forums, though that too is configurable.
An administrator is someone who has complete control over the software that runs the board (vBulliten).
Other groups exist and can be made. For example if someone gets banned they go into the "Banned" group, and that will show up under their name.
HTH,
GAD