AMX Mod is a Metamod plugin that allows you to write scripts for Half-Life and its Mods, using Compuphase's SmallC (what is now called Pawn) scripting language.
It lets a programmer:
For this you must know the SmallC language, and some of the more basic functions programmed into AMX's core. Right now there are already some great plugins ready to download and use, in addition to basic plugins provided in the archive. You can extend AMX functionalities by writing your own modules, that will provide additional native functions to be used in plugins. AMX Mod is simply called AMX (Abstract Machine eXecutive), it is known as that simply due to the filename extension given to the plugin files.