Part of a ballistic missile defense system, an anti-ballistic missile (ABM), is a guided missile, usually fired from a land or ship launcher, that is capable of destroying a ballistic missile. It may attack its target during the boost phase, midcourse, or terminal phase of its trajectory.
ABMs are not the only ways to destroy a ballistic missile. Directed energy weapons could work in all phases; the most active work is on boost and terminal phase lasers. Autocannon or other non-rocket mechanisms (e.g., "Metal Storm") may have a role for point defense in the terminal phase.
All midcourse interceptors are "hit to kill" (HTK), which destroy themselves and their target by the immense kinetic energy release of a collision between supersonic and hypersonic vehicles. HTK is necessary because midcourse intercepts take place in the vacuum of space, where no significant pressure wave would develop if the interceptor used an explosive warhead.
Terminal phase interceptors
This is the most common form of ABM, especially against reentry vehicles of missiles with less than intercontinental range, which fly more slowly than ICBM warheads. Most warheads are HTK, especially those that will intercept above most of the atmosphere.
Different missiles intercept at various altitudes and ranges, and ballistic missile defense systems may very well have layers of defenses, starting by attempting the intercept while still above the atmosphere, and then using faster, shorter-ranged missiles to kill the warheads missed by the original interceptor. Even in a layered defense, it is common to send at least two interceptors at each incoming rentry vehicles.