OpenSim is an open-source software platform that allows users to create and manage virtual worlds. It is often used for educational purposes, simulations, and social interactions, enabling users to build 3D environments similar to those found in Second Life. The platform supports various features, including customizable avatars, scripting, and the ability to import 3D models. Users can host their own servers, giving them control over their virtual spaces and the ability to connect with others in a shared online experience.