Difference between concatenation and appendation

[WhatMeWorry via Digitalmars-d-learn]

Ok, I just made up that word. But what is the difference between 
appending and concatenating?  Page 100 of TPDL says "The result 
of the concatenation is a new array..." and the section on 
appending talks about possibly needing expansion and reallocation 
of memory.

But I still don't feel like I have a grasp on the subtleties 
between them. Can someone give a short and sweet "rule of thumb"?

It might be so obvious that I'll regret posting this.

Thanks.