don't confuse the guy - mono and Microsoft .NET are the same thing - they're both implementations of .NET technology.
Difference between them is that:
* Microsoft .NET Framework works only on Windows
* MS .NET Framework has .NET-standard things faster, because MS decides what goes into next implementation of .NET
* Mono works on many many platforms - including Windows, Linux, MacOS and more
* Mono develops a little slower
* Mono is more wide - it implements things from MS .NET [to be compatibile] and it's own stuff [like things from Linux]

Writing app in/for Mono is basicly the same - the difference is that it doesn't always support everything that's implemented in MS .NET.