Plugins are user-made addons which add to or change the behavior of KSP. They consist of one or more DLL files written in the C# programming language. Plugins allow changing how the game itself works, anything from showing new information windows to making a hovercraft part, or even new planets. Plugin support was added to KSP in version 0.14, and is available on all operating systems.
To use a Part Module Plugin, just drop the DLL into KSP/GameData/ or a new sub folder inside it. To stop using a plugin, just delete it or move it outside the KSP/GameData/ folder.
Plugins can be written in any text editor and compiled with a standalone compiler, but the easiest way to start coding is to use an Integrated Development Environment or IDE for short. An IDE is a program that helps you to write code (in our case the language C#). Listed here are some of the most popular options.
|Visual Studio||Commercial||✓ Yes||× No||× No|
|MonoDevelop (aka Xamarin Studio)||LGPLv2 / MIT (Free)||✓ Yes||✓ Yes||✓ Yes|
|SharpDevelop||Open-source (Free)||✓ Yes||× No||× No|
See the above links for setting up your chosen IDE for building KSP plugins. Once you're ready to build, try creating your first module.