RES
A Ressource (.res) file is a text file that list all custom texture, material, and model files.
It tells the client that he will load certain files from the server. You can edit it with any text editor.
The .res files was often used on the Goldsource Engine to share custom content in maps. The Source Engine is supporting .res files too, but including the custom files wich a programm like Pakrat or PackBSP is a easier and space friendly method.
Creating a .res file
Create a new file and name it exactly like your map, with the extension .res instead of. bsp. And then add all sounds, models, sprites and textures with path, starting from the directory of the game or mod.
testmap.wad testmap2.wad sprites/testmap/glow.spr models/barney.mdl maps/testmap.res
Write the maps/testmap.res into the .res file, then people which downloading the map from a server can share the custom files.
GUIs
A tool called "RESgen" can create automatically .res files by dragging the map into the programm icon.