A common pitfall/gotcha to new C# programmers is the idea of reference types and value types. For a full comparison follow this link: http://www.albahari.com/value%20vs%20reference%20types.html.

For more information individually about the two, please visit the following two MSDN pages:

Value Types
Reference Types

Understanding the differences between the two will greatly increase your ability to decide whether a struct or class is more appropriate when creating new types as well as helping you understand how/why certain types behave the way they do.