Hammer Tutorial V2 Series #14 “Env_Cubemaps, Static Reflections, Reflective Textures”
Env Map Information:
https://developer.valvesoftware.com/wiki/$envmap
Env_Cubemap Information:
https://developer.valvesoftware.com/wiki/Cubemap
Cubemap Weapon Information:
https://developer.valvesoftware.com/wiki/Weapon_cubemap