Unity 3 is an integrated authoring tool for creating 3D video games or other interactive content such as architectural visualizations or real-time 3D animations. Unity is similar to Director, Blender game engine, Virtools or Torque Game Builder in the sense that an integrated graphical environment is the primary method of development.
Simple

With Unity 3, we've put tremendous effort on world building tools пїЅ so you can assemble and tweak your levels at the speed of thought.
Edit. Test. Play. With Unity, testing your latest idea is never more than a click away. This lets you drill down to find the fun, then polish it until it's perfect.
Target platform can be switched instantly, so you can develop from a single source for mobiles, web, PC/Mac, and consoles.

Powerful

We rewrote Unity's rendering pipeline so your games can look the best they can пїЅ and get a 50% performance boost on top.
Unity 3 brings Beast lightmapping and Umbra occlusion culling to the table пїЅ a key part of making games look great and run fast across all devices.
Design the perfect aural ambience with our new integrated audio filters and in-scene editing tools. Captivating your audience has never been easier.

Mac OS X:
Intel CPU & "Leopard" 10.5 or later. Note that Unity was not tested on server versions of Windows and OS X.
Graphics card with 64 MB of VRAM and pixel shaders or 4 texture units. Any card made in this millennium should work.
Using Occlusion Culling requires GPU with Occlusion Query support (some Intel GPUs do not support that).
The rest only depends on the complexity of your projects!.

System Requirements for Unity iOS Authoring
An Intel-based Mac
Mac OS X "Snow Leopard" 10.6 or later
The rest only depends on the complexity of your projects!

System Requirements for Unity-Authored Content
Windows 2000 or later; Mac OS X 10.4 or later.
Pretty much any 3D graphics card, depending on complexity.
Online games run on all browsers, including IE, Firefox, Safari, and Chrome, among others.
Android authored content requires devices equipped with:
Android OS 2.0 or later
Device powered by an ARMv7 (Cortex family) CPU
GPU support for OpenGLES 2.0 is recommended

http://unity3d.com/unity/preview/notes