A is ADMIN, they have control and power over everything beyond our imagination.
U is UBER moderator (yes, really), they have the power to hide questions, opinions, comments; re-align question topics; and also to freeze users. To be fair, it's difficult to come across problems that actually need freezing beyond spam bots, I haven't run into it yet. If anyone is harassing you, ask an uber mod or preferably an admin about it.
S is SUPER moderator, they're like uber mods but they can't freeze accounts (freezing means preventing them from posting, by the way).
M is MODERATOR, they're kinda like super mods except they can't hide questions.