From CloudModding OoT Wiki

Though each version of Zelda 64 is just one large ROM file, each has thousands of smaller files within, each which are deserving of a filetype classification. However for the purposes of this wiki, files should not be referred to using these file extensions.

Scenes

Main article: Scenes and Rooms

These files are what store the environment variables for different levels. Cinematics, collision data, skybox, room files etc.
Labeled by ZRE as "zscene".

Rooms

Main article: Scenes and Rooms

Rooms (also known as Maps) contain geometry, textures, and actors for levels. They also contain brightness settings for individual rooms. If you have a room file without a scene file, they can be easily loaded with another scene file. The level, possibly missing a few textures & no collision, will almost always load.
Labeled by ZRE as "zmap".

Overlays

Main article: Overlays

These files contain the code (ASM/machine code) and/or data which the engine can load to a dynamic location. Overlays are prefixed with the string "ovl_" within the game's file table.
Labeled by ZRE as "zactor", though some overlays are not actors.

Actors

Main article: Actors

Actors are the interactive entities in the game. All actors are overlays, but not all overlays are actors.
Labeled by ZRE as "zactor".

Objects

Main article: Objects

These files contain animation, geometry, and textures for actors. The game uses these files in conjunction with actors to work with NPCs. An extremely useful tool for viewing object files is cooliscool's Utility of Time.
Labeled by ZRE as "zobj".

Text

Main article: Text Format

These files contain text banks from the game. They are decompressed in the ROM for quick access. Generic text aside, there are also different text codes stored within this (color, pauses etc).
Labeled by ZRE as "ztxt".

Code

Main article: Code (File)

Main code block. Due to the specific nature of "code", the checks search for a specific file rather than a file type. Within these blocks, there is usually debugging information and, most importantly, filenames. Majora's Mask and Ocarina of Time both contain filenames of objects and symbols in their main data block.
Labeled by ZRE as "zasm".

Skyboxes

Main article: Skyboxes

Skybox files begin with the "VR_" prefix. Each skybox consists of two files, a vr_*_static file which contains the skybox's textures, and aa vr_*_pal_static which contains the palette for the texture.
Labeled by ZRE as "zdata".

Others

"Unidentified" data. Animations, skyboxes, icons, textures or audio files.
Labeled by ZRE as "zdata".