Difference between idioms, proverbs and metaphors

I am an Indian, and English is my second language. Inspite of tossing a few odd ones every now and then in my writing as well as in speech, I must admit I sometimes do wonder about the difference. This is like one of those obvious amenities, taken for granted in any sphere of knowledge to be implicitly understood, those that we use overly, but do not so fully claim knowing over their technicalities. So I ask, what's the difference between idioms, proverbs and metaphors?