Lasers feature light that all crests and troughs line up. All of the light is the exact same color which is called monochromatic. Lasers do have another unique characteristic; the waves all travel in the same direction and are much more orderly then conventional forms of light. It makes the laser light special in a way that they are uniform in style and have the discipline of the military, the characteristics act as one. Think of it as particles of light all marching in beat like a giant parade. Because they are so disciplined that can easily be controlled and commanded. That is the exact reason they can be used with so many applications.
If the light of the right color hits an atom, it will raise the energy level, when the electron falls back down it gives off light in the same color but in a random direction. When a photon hits an atom that has had the energy raised the atom will let go of its photon. Einstein figured out that when a photon hits an excited atom it releases a new photon that is completely uniform to the incoming photon and travels in the same direction; that's what makes lasers possible. It is called stimulated emission. Basically it is the same concept as cloning.
Laser stands for 'light amplification by stimulated emission of radiation'. Even though we talked about lasers being particles they can be represented as waves. So when the incoming electromagnetic wave hits the atom it and takes the extra energy it has to make energy the wave bigger and stronger.
You might be wondering where the first proton comes from? If we need a photon of each color for each photon then eventually we can't win this is why Einstein could not actually create a laser. But if we get a whole bunch of photons together and excite them without hitting them with photons then it could start a giant chain reaction. It can be achieved by releasing into our atoms or shining different colored light on them. Both of these processes raise the energy level of the atoms and when hey drop down under special conditions they can accumulate. This means it can send off more atoms as they crash together. Now they just need to be organized and travel in the same direction to make a laser.
We use mirrors to bounce the energy back and forth and create a massive amount of photons very quickly. As they bounce back and forth they also become more uniform just like we said about a marching parade. This gives us the population inversion we need to be able to create the laser.
A laser would not be very useful if the entire laser light was held inside in order to let some of the laser out an only partially reflective mirror is used on one side and it lets out the laser light. So even though the laser beam emitted is very powerful, the laser light held inside the unit is much stronger. Thus you have a working laser.