Bush's Missle Shield????

I'm not going to "shoot" this one down (pun intended), but I want to know exactly why this is needed!!??? Now Putin said he could put stuff in Azerbaijan, but Bush said they'd have to look at that proposal. Now he's going on about putting stuff in Poland.

Yes, I understand it's complicated triangulation, but some of us would like to know exactly whose missles we would be defending against and where they are going.

I heard Iran mentioned (again). Yet I hear others say they are 10 years away from developing such technology. And would this "shield" be able to prevent missles from hitting Israel?