What is the Multiple Independently Targetable Reentry Vehicle (MIRV)?

  The MIRV is a ballistic missile which has multiple warheads. Each warhead can be independently programmed to strike a target which is different from all of the other targets so this missile provides more bang for the buck.

The MIRV was a way to get around the limitations placed on the number of ballistic missiles that we could have by the SALT treaty.

The SALT treaty limited the number of missiles but not the number of warheads that could be added to each missile. 

Multiple warheads were an ingenious answer because they provided a more efficient delivery mechanism for nuclear missiles and wee just as effective as using more missiles if not more.  Military Ring Express