Unity is a free game engine. It comes with all the functionality a game developer need to create great games. Pro version comes with added features. Unity allows you to build rich 3D games with animated characters, sizzling graphics, immersive physics. You can deliver your games to the web or as standalone players.
Unity Web Player enables you to view 3D content created with Unity in your browser. Once web player is installed, it updates automatically. Firefox may show plugin as outdated as it does not recognize it yet. Users can not update it, although new version can be downloaded from Unity 3D website.
Unity Web Player for Windows is available for Internet Explorer, Firefox, Chrome, Safari, Opera and Unity Web Player for Mac OS X is available for Safari, Firefox, Chrome. Linux support will be added in version 4. Currently supported platforms are Windows, Mac, Unity Web Player, iOS, Android, Nintendo Wii, PlayStation 3, Xbox 360. They have announced supported platforms in upcoming releases which are Adobe Flash Player, Linux, Windows 8, Windows Phone 8, Nintendo Wii U.
Unity Plugin and Unity Engine are separate. Engine is used to create games while plugin is used to view.