A missile (British English: "miss"-"isle"; U.S. English: "missl") is, in general, a projectile—that is, something thrown or otherwise propelled. Missiles can range from a rock thrown from a slingshot, through a crossbow or ballista bolt, to a Minuteman III intercontinental ballistic missile (ICBM) with multiple nuclear warheads. Modern ICBMs, the largest missiles currently deployed, represent the most destructive weapons ever made in human history.
Rocket-powered missiles are known as rockets if they lack post-launch guidance or missiles or guided missiles if they are able to continue tracking a target after launch. Cruise missiles typically use some form of jet engine for propulsion.
Missiles are often used in warfare as a means of delivering destructive force (usually in the form of an explosive warhead) upon a target. Aside from explosives, other possible types of destructive missile payloads are various forms of chemical or biological agents, nuclear warheads, or simple kinetic energy (where the missile destroys the target by the force of striking it at high speed). Sometimes missiles are used to deliver payloads designed to break infrastructure without harming people. For example, in the Gulf War cruise missiles were used to deliver reels of carbon filament to electricity stations and switches, effectively disabling them by forming short circuits.
Missiles which spend most of their trajectory in unpowered flight, and which don't use aerodynamics to alter their course, are known as ballistic missiles (because their motion is largely governed by the laws of ballistics). These are in contrast to cruise missiles, which spend most of their trajectory in powered flight.
Missiles that have the ability to maneuver through the air can be guided, and are known as guided missiles. These have three key system components:
A tracking system locates the missile's target. This can be either a human gunner aiming a sight on the target (remotely from the missile) or an automatic tracker. Automatic trackers use radiation eminating from the target or emitted from the launching platform and reflecting back to it from the target. Passive automatic trackers use the target's inherent radiation, usually heat or light, but missiles designed to attack Command & Control posts, aircraft or guided missiles may look for radio waves. Active automatic trackers rely on the target being illuminated by radiation. The target can be "painted" with light (sometimes infrared and/or LASER) or radio waves (radar) which can be detected by the missile. The radiation for the painting can originate in the missile itself or may come from a remote station (for example, a hilltop gunner can illuminate a target with a LASER device and this can be used to direct an air launched guided missile).
A guidance system takes data from the missile's tracking system and flight system and computes a flight path for the missile designed to intercept the target. It produces commands for the flight system.
The flight system causes the missile to maneuver. There are two main systems: vectored thrust (for missiles that are powered throughout the guidance phase of their flight) and aerodynamic maneuvering (wings, fins, canards, etc).
There are some similarities between guided missiles and guided bombs. A guided bomb, dropped from an aircraft, is unpowered and uses aerodynamic fins for forward horizontal maneuvering while falling vertically.