Could anyone please explain to me the differences between "diegetic," "non-diegetic," "spatial," and "meta" user interfaces in terms of how they are represented in games and game development?
There is an excellent article over at gamasutra which explains it all, but I'll go over what each mean
In terms of game UI, Non-Diegetic is what you would normally associate a typical game UI, an overlay on top of the game. They have the freedom to be completly removed from the games fiction. Non-Diegetic UIs can represent health and ammo or be a hotbar like in World of Warcraft. Non-Diegetic UIs are normally represented two dimensionally.
On the flip side you have Diegetic, which exist in a game world instead of being overlaid onto the game, very much being part of the games fiction. For example a player could press a button to make their character look at their watch to check the time.
Meta UIs are like Non-Diegtic UIs, the difference is that Meta UIs have some way of staying the fiction of the game. A great example is Call of Duty; There is no health bar, but when you are low on health the screen would be overlay with blood to show that you are damaged. Meta UIs are normally represented two dimensionally.
Spatial UIs are again like Non-Diegetic and Meta UIs, but the main difference is that Spatial UIs exist three dimensionally. An example of this is a glowing trail that appears on the floor that the player can follow.
I'm not sure what "spatial" or "meta" mean, but diegetic and non-diegetic are actually terms that originated with film music and mean essentially the same thing in the context of game UI. Diegetic means it's part of the scene, whereas non-diegetic means it's not part of the scene. So a diegetic UI is interface elements that are in the game scene (think the ammo display on Halo guns) whereas non-diegetic UI is buttons that aren't part of the game world (ie. the majority of game interfaces).