Mission Scripting Tools
Introduction
A more in depth introduction to mission coding can be found here, but basically mission coding/scripting is altering the main.scm (and, in SA the script.img file) to run extra game code.
Game code is restricted to a set of commands defined by the game (called OpCodes), which can be used to test and alter certain attributes of ingame objects. Basically, you can do alot with mission scripting (anything done in any of the missions and in between the missions and more), but not everything is controlled by it, it is very difficult to say what is and isn't.
To give you an idea, the OpCode databases (VC and SA) have fairly cleverly named the OpCodes (although there are alot of unknown ones).
Editing
To open the main.scm file, you will need one of the coding tools, the most common of which are:
III tools
VC tools
SA tools
LCS tools
None yet.