efficacy vs. effectiveness

I wondered what the difference was, so I looked it up.

Main Entry: ef·fi·ca·cy
Pronunciation: 'e-fi-k&-sE
Function: noun
Inflected Form(s): plural -cies
: the power to produce an effect

Main Entry: ef·fec·tive
Pronunciation: i-'fek-tiv, e-, E-
Function: adjective
1 a : producing a decided, decisive, or desired effect
1 b : IMPRESSIVE, STRIKING
2 : ready for service or action (effective manpower)
3 : ACTUAL (the need to increase effective demand for goods)
4 : being in effect : OPERATIVE (the tax becomes effective next year)
5 of a rate of interest : [snipped definition I'm not interested in]
- ef·fec·tive·ness noun

Hmm. Still not entirely sure.