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[
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http://ilspy.net/[
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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.