I want to point you to this great post at Neuroskeptic. Here is its ending text:
"The placebo effect" has become a vague catch-all term for anything that seems to happen to people when you give them a sugar pill. Of course, lots of things could happen. They could feel better just because of the passage of time. Or they could realize that they're supposed to feel better and say they feel better, even if they don't.
The "true" placebo effect refers to improvement (or worsening) of symptoms driven purely by the psychological expectation of such. But even this is something of a catch-all term. Many things could drive this improvement. Suppose you give someone a placebo pill that you claim will make them more intelligent, and they believe it.
Believing themselves to be smarter, they start doing smart things like crosswords, math puzzles, reading hard books (or even reading Neuroskeptic), etc. But the placebo itself was just a nudge in the right direction. Anything which provided that nudge would also have worked - and the nudge itself can't take all the credit.
The strongest meaning of the "placebo effect" is a direct effect of belief upon symptoms. You give someone a sugar pill or injection, and they immediately feel less pain, or whatever. But even this effect encompasses two kinds of things. It's one thing if the original symptoms have a "real" medical cause, like a broken leg. But it's another thing if the original symptoms are themselves partially or wholly driven by psychological factors, i.e. if they are "psychosomatic".
If a placebo treats a "psychosomatic" disease, then that's not because the placebo has some mysterious, mind-over-matter "placebo effect". All the mystery, rather, lies with the psychosomatic disease. But this is a crucial distinction.
People seem more willing to accept the mind-over-matter powers of "the placebo" than they are to accept the existence of psychosomatic illness. As if only doctors with sugar pills possess the power of suggestion. If a simple pill can convince someone that they are cured, surely the modern world in all its complexity could convince people that they're ill.