.NET is a Microsoft version of a stack-based virtual machine for running managed code.

At present moment, you can not run .NET natively on any processor (but perhaps see my article on the topic of Hardware support of the current stack-based virtual machines), so you need a .NET framework to run the managed code. For Windows, there is a fully featured and smoothly running 3.0 version available, for Linux there is the Mono project, which is trying to implement equivalent functions to the Linux environment, but as far as I tried, you can't rely on this as being the same as the Windows .NET framework.