SAM is an acronym for
Surface to
Air
Missile. Most of the types used by
North Vietnam during the
Vietnam War were radar guided. The U.S. has several variants, ranging from the
HAWK (
Homing-
All-the-
Way-
Killer. I kid you not) to the
Patriot system. The
Navy uses
VLS variants such as the
Sea Sparrowand the SM-1/2
Standard Missile.
There are several types of radar-guided missiles. Some, such as the AIM-54C Phoenix and AMRAAM (both air-to-air), have onboard radar or other sensors which are used to guide the missile to the target, either starting some time after launch or at launch. Others, like the HAWK, Patriot and Standard Missile, use ground-based radars to not only detect and track targets, but to guide the missile all the way to intercept (Actually, I need to back down; I don't know if the Standard uses onboard radar at any point). These systems take their image of the target and their image of the missile, and send commands to the missile along radio or wire links until intercept. These systems may require additional 'handoff' guidance radars which can spend more 'time' on a target that a missile is heading towards. You will occasionally see smaller radar installations on Aegis ships in addition to the SPY-1D or SPY-2 radar; that's what they're for. They can also be used to communicate with the missile. It is not uncommon for these systems to have very short-range sensors in the missile that take over when they are close enough to the target, freeing the tracking radar for other targets/tasks.
Finally, there are 'passive homing' systems such as Sea Sparrow. In these systems, sensors in the missile 'see' the reflected radar energy from the target that is generated by the main ground-based radar, and use those signals to guide themselves to intercept. The problem with this (especially for aircraft, for whom the Sparrow was originally designed) is that you must illuminate the target with your radar continuously until the missile hits or has definitely lost lock.
Running a ground-based SAM isn't all that safe, either. See Wild Weasel for more.