What is the difference between efficacy and efficiency?

Well, efficacy is whether it works or not... whether it has that capacity. Efficiency is not whether it works, but how well it works... how quickly, how many people have to be involved, etc. If you cut down the amount of time it takes or the amount of workers it takes, then you are improving efficiency.
Here are the links for the words on dictionary.com if you would like to investigate further:
http://dictionary.reference.com/browse/efficacy
http://dictionary.reference.com/browse/efficiency

 

Answer

Efficacy is defined as 'that which brings about the desired result'.


Efficiency, in engineering terms, is defined as 'the ratio of a machine's output power to input power.