Dont change the letter of the .j file.
The war3map.j file : JASS2 Script
This is the main map script file. It's a text file and you can open it with notepad.
Sometimes it's renamed to Scripts\war3map.j by map protectors to keep you away from it.
The language used is called JASS2 and has been developed by Blizzard. It's a case sensitive language.
When you play a map, the jass script is loaded and executed.
When you select a map in when creating a game Warcraft III will first look up the "config" function and execute its code to set up the player slots.
Then, when the game has started, Warcraft III looks for the function called "main" and executes it.
You may find more informations at :
http://jass.sourceforge.net/doc/The war3map.w3i file : The Info File
It contains some of the info displayed when you start a game.