An MIRV (Multiple Independently targetable Re-entry Vehicle) is a bus that can deliver multiple nuclear warheads from a single launch vehicle.
Depending on the launch vehicle, each missile can carry up to ten or twelve separate warheads and decoys, or “chaff.” Each warhead in the MIRV is independently targetable.
During testing, videos and time-exposure photographs were taken of MIRVs during reentry and descent. The results are visually striking.
The LGM-118 Peacekeeper reentry photo is one of the most widely circulated. A lower-resolution version of this picture is featured on Wikipedia.
There are a few videos showing actual reentries, but most are very low quality. The best compilation is this amazing video that must be from the early nineties: Minuteman III Missile Launch & Re Entry- California to Kwajalein Atoll (edit- This was removed youtube, but Dziban provided the Liveleak video below in a comment). It looks like a VHS rip, and it has a great soundtrack:
This throwback LGM-118 MX Peacekeeper ICBM information video goes into depth about the launch and reentry process:
Minuteman Weapon System History and Description (pdf) (2001) FOIA/Declassified