A laser guided bomb is - just like the name of the thing implies - a bomb guided by a laser.
It would seem so, but the misconceptions of this particular type of weapon are many and not so far between. Every time the goings-on in faraway countries call for explanations of the kind of things falling from aeroplanes, exploding in command bunkers, granite caves, TV towers or baby milk factories, the media pulls up some kind of cartoon style graphics explaining how it works. Only they don't. Explain how it works, that is.
What it's not
A laser guided bomb is not a so-called smart bomb. Hopefully, the person in the cockpit fifteen thousand feet up in the air is smart though. If he or she is dumb, then the bomb pulled by gravity towards its final and violent demise is also dumb. That would be dumb. Smart bombs also don't do collateral damage, something years and years of playing Defender have taught me.
A laser guided bomb is not a laser weapon. A news anchor repeatedly called the things "Laser Bombs" throughout the coverage of the 1999 Kosovo conflict on a TV channel I do not wish to mention the name of. A laser guided bomb is not a laser weapon. There are no brilliantly colored lasers beams protruding from the bomb, making smoking craters on the ground. The good old somewhat chemically unstable and explosive trinitrotoluene takes care of that, but not until the bomb has actually hit anything and the fuze have done its job.
A laser guided bomb does not make all wars look like video games. This is what they told you after Desert Storm ended. The neatly combed news anchors with their model girlfriends and digitally-enhanced-for-whiteness teeths. That's who. The video game part stems from the blurry-grainy imagery shown on press conferences. You know, little dots flying around the screen, lots of confusing numbers, a flickering crosshair, the luckiest guy in Iraq and something that the news anchors probably thought was today's high score in the lower right corner of the screen. Hence the term "Video Game". You have all played video games. If video games had the kind of procedures in it that the dropping of a laser guided bomb demands, you wouldn't have bothered playing it. Ever.
A laser guided bomb does not hit its intended target no matter what, sending chunks of hopefully highly expensive debris in all directions. It is not smithereen-safe. The guy (or not) in the cockpit must - no matter what happens and no matter what the news anchors tell you - be absolutely sure that the bomb leaves the wing of the airplane at the right time. You simply cannot force a falling two-thousand pound object backwards by use of gravity and drag. No Can Do. If the bomb's dropped too late or too early, it is outside what air force people call "the envelope". If it's outside the envelope it will miss. And that's not all; a bomb that falls towards mother earth and suddenly no longer sees the laser reflecting off its target will also miss. This can happen if a cloud or something else gets in the way of the laser beam that's supposed to guide the bomb.
A bomb that misses often ends up headlining the nine o'clock news. That's bad; lots and lots of people will be considered dumb - not just the poor bomb.
How it works
First of all, a laser guided bomb is propelled through thin air by the forces of nature old Newton found out of. You know, apples and stuff. They are gravity and drag. Actually, Newton didn't do much on drag research, but that's beside the point. Gravity and drag, those are important to remember though.
The first thing you need for a successful laser guided bomb is a laser. Without a laser you'd have no laser guided bomb. The device that emits the laser beam is called a "designator". The designator's job is to point a laser beam at the thing you want to explode. If you have seen the 1994 movie Clear and Present Danger where Willem Dafoe points an unwieldy binocular-looking device at a cocaine baron's (or is that "evil drug lord's"?) house, you will know what I'm talking about. That's the designator. If you've seen the movie, you know how that scene ended. Expensive debris etc. etc. Designator's aren't always carried around in the jungle by Willem Dafoe. Designators can also be hung from the belly of an aircraft, with or without bombs. This way, a guy (or not) on the ground, the pilot himself or his buddy in the next plane over can "tag" the target. This last method is aptly named "Buddy Lasing". One word of caution: staring into the beam of the lasers like the ones used to guide laser guided bombs will make you go blind. That's why most air forces don't bomb people with laser guided bombs. It's a safety measure.
Next you need a bomb. Any bomb will do, and NATO have plenty of these "any bombs". In military speak they're named Mk-82, Mk-83 and Mk-84. NATO of course have other bomb models beside those, but for the sake of easiness (and the chance of sounding like a news anchor), let's stick to those. The majority of laser guided bombs dropped by NATO around the world are based on those three types. Incidentally they are called "dumb bombs".
Now that you have the bomb and the laser, you'll need some sort of magic to make the bomb concentrate on the place on the target where the laser beam is pointing. This magic device is called the guidance unit. This magic piece of equipment is bolted onto the nose of the dumb bomb we talked about before. The energy of the laser beam reflecting off a target is all this thing cares about.
Okay, we have the bomb, we have the laser and we have the guidance unit in the bomb's nose. Now what? How do we make the bomb go where we want to? This must be the part where the drag comes into the picture? Right. The answer to this question is wings. Yes, huge (and some not-so-huge) wings are fitted to the nose - those are the small ones - and the tail - those are the huge ones - of the bomb, right where the old fashioned fins used to be when the bomb was dumb. The wings are connected by wires to the guidance unit upfront, providing lift and correcting the course of the bomb's gravity-induced voyage to the ground (or tank or house or baby milk factory).
Man in the loop
Before all this can happen, the guy (or not) up in the cockpit have to make some decisions first. Those decisions involves some pretty wild three-dimensional mathematics. The guy (or not) driving the plane must know about his speed relative to the ground, the compass heading he is flying, where the target is in relation to himself, the attitude of his plane (contrary to polular belief, F-15's aren't grumpy in the morning), reigning weather conditions and the airplane's altitude. When the pilot is aware of all these things, he can start deciding when the bomb is going to drop. If he fires up his laser from say 25000 feet up, too far away and then drops his bomb, the bomb will point its nose towards where the laser beam ends and be out of kinetic energy long before it's anywhere near anything worth blowing up. The pilot was in this case outside the laser guided bomb's envelope and well inside the nine o'clock news headlines. That is a bad thing, and that's why you need high school to become a pilot.