It can be used to create a virtual environment (or world) which can be accessed through a variety of clients, on multiple protocols. It also has an optional facility (the Hypergrid) to allow users to visit other OpenSimulator installations across the web from their 'home' OpenSimulator installation. In this way, it is the basis of a nascent distributed Metaverse.
OpenSimulator allows virtual world developers to customize their worlds using the technologies they feel work best. OpenSimulator is written in C#, running both on Windows over the .NET Framework and on Unix-like machines over the Mono framework. The source code is released under a BSD License, a commercially friendly license to embed OpenSimulator in products.
Out of the box, OpenSimulator can be used to simulate virtual environments similar to Second Life, given that it supports the core of SL's messaging protocol. As such, these virtual worlds can be accessed with regular SL viewers. However, OpenSimulator does not aim to become a clone of the Second Life server platform. Rather, the project aims to enable innovative feature development for virtual environments and the Metaverse at large.
So although OpenSimulator is similar to Second Life (closed commercial grid), it is open-source with more freedom to create and build your own creations and you can even run your very own grid for example for simulations for work, or for house design, medical training, etc. There are also open commercial grids like Digiworlds which have their own economy. OSgrid is the largest and one of the oldest grids around, home to a lot of technicians, creative artists, and a lively party community. But Sci-Fi grid is much likely much more awesome for you if you are into star trek and space stuff.
Find out more at http://opensimulator.org/wiki/Main_Page