'Improvised Explosive Device' (IED) is not really a replacement for 'Bomb', but for 'home made bomb'.
I read 'bomb' as meaning any explosive device, and unless specifically stated (home-made, or IED), as something which can be purchased from an arms dealer.
IEDs are more rough and ready, and available to those with whom arms traders would not normally deal.

IED: Not to be confused with IUD :)