If someone could explain these and/or make a thread on them, that'd be great.
Please
Nameplates are a work in progress, but they are identical to the ones in Reach. They're for display purposes only and have no inherent powers.
Ranks are Newbie, Member, Posting Spree, and the like. They're post based.
Prefixes are the things that come before ranks. So you'll have
Respected Member and
Ascended Member at the same time you can have someone with
Bad Posting Spree. These are the cool things that change your userbar color. They are calculated using Trust (how well you post in the forums), but will soon take other variables into account.
Then there are special cases like Admins and Mods, who are out of that system completely. Admins always have red bars and mods have orange. They have no prefix and are immune to trust ratings.
To summarize: it's a mix of the old Bungie ranking system and the Halo Reach nameplate system.