Developers are non administrators who contribute to the engine. They have limited moderation powers, namely the ability to modify posts from other members and write access to the coding area. This is to aid in development discussions (like say, adding additional notes to a fellow developer's release post), not for actual moderation purposes. It's possible to be both a Developer and part of the forum staff (easy example; I am both a Developer and the Site Owner).
Administrators help run the forum. They have have full moderation and forum adjustment powers.
Moderators (we don't have any atm) have moderator powers but cannot make changes to the forum itself.
The Site Owner (yours truly) is obviously the final authority by virtue of actually owning the space and running the site (hence the name). In practice though I delegate to the Admin staff for moderator decisions (otherwise why have a staff?) and will only reverse them with a majority vote or in the most extreme circumstances.
Once upon a time we had local Moderators for specific boards, but this proved useless and we haven't bothered since.
DC