The term "reentry vehicle" means that part of the front section that can survive reentry through the dense layers of the Earth's atmosphere and that is designed for delivering a weapon to a target or for testing such a delivery.
--START, Annex: Terms and Their Definitions, #88 (English) / #8 (Russian)
An RV is the aerodynamic shell in which an ICBM's "physics package" is kept. It is dispensed by the missile or by the missile's SCDM (self-contained dispensing mechanism), a.k.a. the PBV (post-boost vehicle) or the "bus", whereupon it arcs to its apogee tumbling gently, or perhaps spin-stabilized like a gyroscope, and then begins its descent1. At about 100km altitude, it starts to encounter atmosphere, and the heat shielding begins to get hot. There is no nose cone really--the whole thing is conic in shape2. The entire skin (typically a carbon or phenolic composite) gets white-hot, and depending on its ballistic coefficient, it can reach speeds of Mach 20+. Of course, in the upper atmosphere, measuring speed by Mach number is a little silly.
The RV comes in towards its target ballistically, unguided, not changing direction at all3. If you're lucky enough to be standing within 20 miles of the RV as in comes in, it looks just like a big, stunningly graceful meteor. Really, it's just space junk, obeying the law of gravity, just like any good meteor, only it is much more aerodynamic. As the START definition notes, it is "designed to survive reentry." So the meteor trail is much longer, much prettier, and infinitely more ominous. In an inert test, the trail is followed by a sonic boom, and then a mild rumble like thunder or ripping linen. In a live warhead, I don't know whether you'd hear the sonic boom from reentry, or if the flash from the fusion detonation would overtake it before you got to hear it. Then again, it doesn't really matter much, does it?
- It is important that the RV be pointed towards its target before it is spin-stabilized, otherwise the spin will actually cause it to hold an incorrect attitude on re-entry. This results in the RV's sidewalls (rather than the nose tip) receiving the majority of heating, resulting in tumbling, ablation, and eventually failure. An attitude control system or a post-boost vehicle can take care of this.
- In addition to conic shapes, RVs typically have a hemispherical nose tip. Further, the smooth conic side can have an angular break where the shape flares or tapers. Each segment adds to the complexity of the shape but can offer a marginal increase in performance; biconic, triconic, flared triconic, and so forth.
- Unless it is a MaRV, or Maneuvering RV.