File Extensions

You will mostly be editing these.

.decl:decl

The most common file type you will be using. Can be edited with a text editor.
Found in:
<resource>/generated/decls
Example:
gameresources/generated/decls/weapon/weapon/player/rocket_launcher.decl
> File to edit the Rocket Launcher.

.entities:entities

Contains everything in a level except geometry. Edit this when making level mods.
The .entities files are compressed and would require a tool to decompress it. After that, the file can be edited with a text editor.
Found in:
<resource>/maps/game/<campaign>/<level_name>/<level_name>.entities
Example:
e1m1_intro_patch1/maps/game/sp/e1m1_intro/e1m1_intro.entities
> Level file path for Hell on Earth.

.tga:tga

Texture files. Not everything "visual" are contained here as some aspects such as particles and colors are delegated to .decl files.
Texture editing software would be required to edit .tga files.
Found in:
<resource>/models" and "<resource>/art
Example:
gameresources/models/monsters/arachnotron/*
> Texture files for the Arachnotron's model.