The word itself stands for "light amplified by stimulated emission" ... which explains nothing about what lasers actually are, what they produce and how they work. So let's start from the beginning. What's the difference between laser light and "normal" light?

Laser light is coherent and collimated while normal light is incoherent and uncollimated. It's actually not that hard to understand. Coherence is when a band marches in beat. Incoherence is when you have a crowd of people loudly talking in the background. Collimation is when you walk in a straight line in the same direction as everyone else. Uncollimated people like to follow their own path.

Why does this matter? It matters because laser light sticks together while normal light scatters about. Laser light is a tightly coordinated military parade in tiannamen square while normal light is a bunch of drunk guys after a soccer match.

If I want to send a signal from A to B in 10 seconds, laser light will take it from A to B in 10 seconds. Normal light will take it from A to B and C and D. Some pieces will arrive in 10 seconds, others at 11 seconds, some even at 12.

Double hetero-junction diode lasers:

This image is an electron microscope photo taken on the cross-section of a double hetero-junction diode laser. Despite the name sounding really complex, the concept really just consists of sandwiching one material in between another material. It's a cheese toast in essence. The "cheese" is InGaAsP (Indium Gallium Arsenide Phosphide) and the "bread" is InP (Indium Phosphide). How does it work? The bread and cheese are designed such that light and electricity get squeezed into the cheese and don't leak out of it. The laser light travels horizontally through the cheese layer. So you just put electrical energy into this thing and it squeezes out light like a toothpaste bottle wherever you give it an opening.

And yes, you're seeing this right. The part of the laser that produces light (active layer) is 1.3 microns, or 50 times thinner than human hair. Humanity taught a thing sheet of rust on metal how to think and used that power to make gooner slop. It brings a tear to my eyes.

Dye lasers:

You see that tube with the red liquid on the left side? It's carrying rhodamine 6G, a fluorescent dye. Ya shine light onto it and it glows yellow. You can see the yellow glow in the center window thing, the upper right corner and in the lower left. You can also see the remnant fluorescent glow as the rhodamine is pumped out through the right tube.

Do I have anything interesting to say about dye lasers? Uh, they can cool themselves. Cause the thing producing light (aka the dye solution) is pumped out. So that's neat. You can also just read the wiki cause I know little about these types of lasers.

Pulsed lasers with nonsensically high power outputs:

Some lab in Romania genuinely made a laser with 10 petawatts of power. These days you also have lasers which create femtosecond pulses (1 femtosecond is 1 billionth of a nanosecond). In fact, the other day, I did a lab that used femtosecond lasers. My group mate was waving his hand through the laser and we were collecting data on how transparent his hand was (spoiler, his hand wasn't transparent). Uh ... that goes against the laser safety lesson that comes right after .... forget I said anything!

Anyway, you wonder how they make these ridiculous lasers? They do it by forcing all the power of the laser into short pulses and releasing it at once.

It's like basically pressure cooking the laser medium and releasing all the energy in a whistle. This technique is called "Q-switching" (think of "Q" as the energy retention factor. You make it go up to store energy then release). This Q factor technique however only gets you so far. To make real femtosecond lasers you need to do this thing called "mode-locking" which I'm not going to explain.

Just read more at

https://www.rp-photonics.com/q_switching.html

https://www.rp-photonics.com/mode_locked_lasers.html

It's just a miliwatt laser, what's the big deal? I have a 60 watt light bulb in my house and I don't need any special light bulb safety training. Well remember how laser light goes "from A to B" while normal light goes from "A to B and C and D"? Laser light is amazing when "point B" is a communication device. You're getting the maximum power of the light focused onto the device for it to pick up. That's not what you want when "point B" is a spot in your retina. You don't want maximum power focused onto a spot. You want it spread out.

Not to mention, lasers are beams, so the whole power of the laser goes into your eye or onto your skin instead of like, 1% (cause the rest spread out to other parts of the room)

Lasers come in 4 classes (with some subclasses)

Class 1: Harmless under normal use

Class 2: Your reflexes will protect you if it gets into your eye

Class 3R: Harmful if it gets into your eyes

Class 3B: Even reflections and exposure to skin is harmful

Class 4: Can even set fire to things

Classes 1 and 2 also have a special "1M" and "2M" subclass, which means "it's harmful if you focus the beam into your eyes through some lens".

The general advice for handling lasers is

1. Don't bring reflective or flammable materials into the room
2. Use specialized laser safety goggles
3. Don't enter a room with a turned on laser without authorization (rooms with lasers should have a clear "LASER ON/OFF" sign)
4. Don't lean over to get a better look at the laser. The laser should remain below your eye level so it doesn't accidentally get into your eyes
5. Keep the emergency number on speed dial

These rules can be relaxed for low class lasers and if the laser is contained inside a fiber, box or other such system.

A source on the medical effects of laser exposure