NSIS, or Nullsoft Scriptable Install System, is an open-source tool used for creating Windows installers. It allows developers to package their software applications into a single executable file, making it easier for users to install programs on their computers. NSIS is highly customizable, enabling developers to define installation processes, add components, and create user interfaces. The scripting language used in NSIS is simple and flexible, allowing for various installation scenarios. Developers can include features like file compression, registry modifications, and shortcuts. Due to its lightweight nature and extensive community support, NSIS is a popular choice for software distribution among developers.