The Sparrow and Standard are "semi-active radar homing missiles", meaning that they have a radar seeker head and no emitter. They require an external illumination radar (from the aircraft or launch platform) to light up their target.
The AIM-120 uses a datalink to guide the missile close to the target, after which the missile's internal radar emitter goes active, and the missile locks onto the nearest target that it finds. This is because the radar on the missile is not nearly as powerful as the radar on the aircraft. So it's not truly a "fire and forget" missile, at least if a high probability of kill is desired. I believe the Phoenix is similar.
