Fire-and-forget is a type of missile guidance which does not require further guidance after launch such as illumination of the target or wire guidance, and can hit its target without the launcher being in line-of-sight of the target. This is an important property for a guided weapon to have, since a person or vehicle that lingers near the target to guide the missile (using, for instance, a laser designator) is vulnerable to attack and unable to carry out other tasks.
Generally, information about the target is programmed into the missile just prior to launch. This can include coordinates, radar measurements (including velocity), or an infrared image of the target. After it is fired, the missile guides itself by some combination of gyroscopes and accelerometers, GPS, organic active radar homing, and infrared homing optics. Some systems offer the option of either continued input from the launch platform or fire-and-forget.
Fire-and-forget missiles also have the tendency to be deflected by soft-kill systems on modern main battle tanks as well as also getting destroyed by hard-kill systems. As opposed to unguided RPGs which require a hard-kill system (a counter projectile(s) used to destroy the incoming missile), fire-and-forget missiles can often be jammed by means such as electro-optical dazzlers.
Many of these are infrared homing missiles; some of the remainder (e.g. AIM-120) are active radar guided.