NSIS (Nullsoft Scriptable Install System) is an open-source software that allows developers to create Windows-based installers. It supports various installation types, including full, minimal, and custom installations. NSIS also provides a powerful scripting language that enables developers to customize the installation process and add custom actions, such as creating shortcuts or registering DLLs. Additionally, NSIS offers advanced compression and encryption options to ensure secure and efficient software distribution.