In GoldSrc,
materials.txt is a file found in a game/mod's sound directory. It is a list of entries consisting of pairs of texture name and a single letter code specifying the material of that texture. These materials make surfaces with textures matching an entry emit different sounds when walked on and when hit with a crowbar.
How texture names are read, and what materials are available, differ between games/mods. In general, texture names discard special prefixes (
{
,
!
,
+#
,
-#
,
~
, etc.) and matches up to 12 characters. Two notable exceptions are Counter-Strike and Day of Defeat which matches the full texture name, including the prefixes.
Some example materials include tile, vent, dirt, computer parts, and glass. Again, some games/mods expands on these entries.
Note that the list is singular and global, complicating the process of making custom map with custom textures.
Tutorials: