What is the difference in managed and unmanaged code, memory and size?

After seeing and listening a lot regarding managed and unmanaged code, and knowing the only difference is that managed is about CLR and un-managed is outside of the CLR, it makes me really curious to know it in detail. What is it all about, managed and unmanaged code, memory and size?

How can code I write in C# be unmanaged while this is C# code and how does a memory of size becomes unmanaged. An example and a little insight would be helpful.