Next to members' names you will see their "rank" and "group".
The "rank" is purely for fun and makes no difference to your use of the Forum. Like a soldier, you start as a Private and get promoted up the ranks. The more posts you make, the higher you go. So all those Major-Generals you see are not senior army officers, they are just ordinary members who happen to have been around a while and piled up a lot of posts.
The "group" is different. You enter in "Members1", which allows you access to certain parts of the Forum, add messages etc. Once you have posted five times, you get moved automatically into "Members2" which brings you more privileges such as more storage space for private messages, ability to search more often, etc. Finally, when you get to 100 posts you become one of the "Old Sweats" and you automatically gain access to an area where you can relax and discuss off topic items.
There are also specialist groups for the "Admin" team.