Not much. Auto implemented properties are simply syntactic shorthand for the compiler. If you look at the assembly using a tool like ILSpy, you'll see that an auto implemented property actually gets converted to property + backing field at compile time.http://msdn.microsoft.com/en-us/library/bb384054.aspx
The only real-world difference is that a manual property can do things with the value before it is assigned or returned. An auto property just sets or returns the value.