OGRE (Object-Oriented Graphics Rendering Engine) is a scene-oriented, flexible 3D engine written in C++ designed to make it easier and more intuitive for developers to produce applications utilising hardware-accelerated 3D graphics. The class library abstracts all the details of using the underlying system libraries like Direct3D and OpenGL and provides an interface based on world objects and other intuitive classes.
It's ok if you take them by their word and look at it as a render engine, but the website etc really makes you want to use it to make games. That's where the problems begin though. First of all to set it up and compile it is a major pain. Then you have to get addons to have any form of input from keyboard and mouse and then you have to get a 3rd party GUI.
Once you're past that, you try to get something going but most of the time, Ogre sort of fights back and you dont really know how to do what you want to achieve. The documentation is pretty much useless and the forums aren't much help either. Most of the time you either get no answer or just a "have a look at the code".
Ever since the main developer retired from the project there is not much progress. They are discussing future steps endlessly in the forum, form subcommittees to decide this or that and so on.
On top of it all, the Ogre concept is outdated and more aimed at a fixed function pipeline. Shader rendering leaves much to be desired and is slower than it has to be.
All in all, it's OK for a free engine, but I wouldnt really go with this anymore.