RuneScript was created by the gower brothers
RuneScript is a scripting language that Jagex uses to create content for RuneScape. The game engine is not written in RuneScript, but instead Java. RuneScript was developed by Jagex Ltd. to allow their other staff that do not know Java to add and edit content for the game. The game engine reads RuneScript files and executes them accordingly. This is the structure most larger programs (such as other video games) follow.
This scripting language is similar to the markup used for articles on this site. RuneScript is parsed into Java for the computer to read just as certain syntax in RSPS markup is translated into HTML.
The minor updates are usually for introducing a new map and adding new NPCs, and for these updates, a RuneScript file is added into the program. The major updates are for the source code, which is usually for making large changes such as introducing new areas in RuneScape, new skills, or for bug fixes.