puggsoy's descriptions are pretty spot on but I'll list out things a bit more exactly for the sake of clarification.
Site Staff - Have edit ability across all of the main sites. This includes the actual content (sheets, models, textures, and sounds) and everything related to them (icons, submitters, etc.), games (including their icons, the console they belong to, etc.), sections (order and name), and all comments. The only thing they can't edit are consoles. They also have access to the currently incomplete systems required to mass-add tags and genres. And of course, they can manage the submission queue and approve/reject submissions, games, and revisions. With the ability to process submissions also comes instant approval of their own. Site Staff have no powers on the forum.
Site Admins - Everything Site Staff can do plus a handful of additional tools including the ability to directly create and edit consoles, rename and merge submitters, access the site's activity logs, and of course post updates to the home page. Their submissions are also instantly approved. Site Admins also do not possess any forum abilities.
Moderators - This is where things differ a little. By default, Moderators do not have any access to the sites' control panels. Their powers are limited to the forum and include full moderation abilities in all areas. I'm sure I don't need to go into detail with what that entails and puggsoy's description of the forum abilities is already pretty complete. For reference, Vlad was a member of the Administrators group but Moderators do have limited abilities to edit profile fields, avatars, and signatures. If a Moderator is also a member of the Site Staff group, their abilities on the sites will match those of the normal Site Staff. If they are additionally in the Site Admins group, they'll have full Site Admin powers on the main sites as well (Mighty Jetters, for instance, falls into this latter category).
Administrators - Administrators also have full moderation capabilities across the entire forum (the exact same tools as Moderators) but additionally have access to the forum's Admin Control Panel which can be used to modify many aspects of the forum (settings, sections and forums, users, templates and styles, etc.). I have Administrator permissions somewhat locked down to remove the ones that general staff wouldn't need access to but, for the most part, the Administrators can do just about anything and everything on the forum. As with Moderators, by default, Administrators have no access to the sites' control panels. The difference here is that if they are assigned access to the sites, they automatically get full Site Admin access. This is also somewhat of a formality as forum Administrators can edit all aspects of a user, including their group, and could therefore assign themselves access to the sites if they really wanted to. Everything else puggsoy said about this group was correct.
As puggsoy assumed, Dazz and I both have unrestricted access to everything at all times. None of the Administrator group permissions apply to either of us and we also have access to some restricted features on the main sites which I won't go into here but they're not all that interesting anyway. Generally, there's not too many reasons for a lot of people to have this kind of access to the forum which is why we are the only members of the Administrators group.
I hope this answers your questions. I'm actually somewhat surprised this has never been asked before given that we've had this staff structure for quite a while now. If you are curious about anything else, feel free to ask. And puggsoy, thank you for writing that up - despite not having access to most of the features you described, your explanations were really very accurate.
Site Staff - Have edit ability across all of the main sites. This includes the actual content (sheets, models, textures, and sounds) and everything related to them (icons, submitters, etc.), games (including their icons, the console they belong to, etc.), sections (order and name), and all comments. The only thing they can't edit are consoles. They also have access to the currently incomplete systems required to mass-add tags and genres. And of course, they can manage the submission queue and approve/reject submissions, games, and revisions. With the ability to process submissions also comes instant approval of their own. Site Staff have no powers on the forum.
Site Admins - Everything Site Staff can do plus a handful of additional tools including the ability to directly create and edit consoles, rename and merge submitters, access the site's activity logs, and of course post updates to the home page. Their submissions are also instantly approved. Site Admins also do not possess any forum abilities.
Moderators - This is where things differ a little. By default, Moderators do not have any access to the sites' control panels. Their powers are limited to the forum and include full moderation abilities in all areas. I'm sure I don't need to go into detail with what that entails and puggsoy's description of the forum abilities is already pretty complete. For reference, Vlad was a member of the Administrators group but Moderators do have limited abilities to edit profile fields, avatars, and signatures. If a Moderator is also a member of the Site Staff group, their abilities on the sites will match those of the normal Site Staff. If they are additionally in the Site Admins group, they'll have full Site Admin powers on the main sites as well (Mighty Jetters, for instance, falls into this latter category).
Administrators - Administrators also have full moderation capabilities across the entire forum (the exact same tools as Moderators) but additionally have access to the forum's Admin Control Panel which can be used to modify many aspects of the forum (settings, sections and forums, users, templates and styles, etc.). I have Administrator permissions somewhat locked down to remove the ones that general staff wouldn't need access to but, for the most part, the Administrators can do just about anything and everything on the forum. As with Moderators, by default, Administrators have no access to the sites' control panels. The difference here is that if they are assigned access to the sites, they automatically get full Site Admin access. This is also somewhat of a formality as forum Administrators can edit all aspects of a user, including their group, and could therefore assign themselves access to the sites if they really wanted to. Everything else puggsoy said about this group was correct.
As puggsoy assumed, Dazz and I both have unrestricted access to everything at all times. None of the Administrator group permissions apply to either of us and we also have access to some restricted features on the main sites which I won't go into here but they're not all that interesting anyway. Generally, there's not too many reasons for a lot of people to have this kind of access to the forum which is why we are the only members of the Administrators group.
I hope this answers your questions. I'm actually somewhat surprised this has never been asked before given that we've had this staff structure for quite a while now. If you are curious about anything else, feel free to ask. And puggsoy, thank you for writing that up - despite not having access to most of the features you described, your explanations were really very accurate.