What is the difference between in-proc and out-of-proc? What technology enables out-of-proc communication in .NET?

Purpose:

• Remoting as a topic is rather old and as such a question like this could illustrate the depth of experience or a good understanding of how distributed software development has evolved over the years.
• A discussion of the performance penalty of marshalling and going out-of-process would be a good sign.

Potential Answer:
This is referring to .Net Remoting which allows for method invocation across application domains. It provides an abstract approach to inter-process communication. in-proc is when you are in the same application domain and is faster since out-of-proc requires marshalling and serialization between the two process boundaries.