What is the purpose of ~' or '~ in Clojure?

I am learning about Clojure macros, and the code examples will sometimes have the constructs '~symbol or alternately ~'symbol. I know that (quote and ' prevent a form from being evaluated, and that the backquote additionally adds namespace qualification, and that ~ causes a quoted form to be evaluated. My question is: why is it useful to stop then start evaluation? I also assume that ~'symbol and '~symbol are different, but how so?