Without getting technical and confusing. The difference between semi-auto and a fully automatic weapon is a semi auto will only fire 1 round every time you pull the trigger. If you pull the trigger an hold it on a semi auto, it will only fire once, when you release the trigger, it will "reset" and allow you to pull it again to fire another single round.
A fully automatic weapon, while being illegal in many places and/or requiring special licensing/permits, will fire multiple rounds, for as long as the trigger is held down. When you pull the trigger and don't let go, it keeps sending lead down range.
Both either use the recoil of the weapon to load a new round into the chamber or they use the gases created by the ignited gunpowder and convert it into physical energy required to reload the weapon also referred to as direct gas impingement or mechanical impingement.
Say you were shooting something like this:
This weapon uses recoil (the backward pressure of the gasses used to drive the projectile, or bullet, out of the barrel) to drive the slide back and a spring to push it forward thereby ejecting the spent casing and loading a fresh round into the chamber. While shooting a gun like this, if there isn't enough pressure from the palm of your hand (supported by your shooting stance and grip) The slide will not be able to operate properly. The entire gun will move (also known as muzzle flip and "kick") rather than allowing the slide to operate properly.
The advantage to a gun like the semi-auto over a revolver type, is in the reloading and the mechanical moving parts. The semi-auto handguns are more complex (moving parts wise) than a revolver. The semi-auto also uses a magazine to hold the bullets as opposed to loading them into the "cylinder" of the revolver. This makes it easier/faster to reload your gun and also allows you to carry extra magazines.
Please note there is a difference between a magazine and a clip (referenced in a video I recently posted of a grossly misinformed politician).
As to the "loudness" of them being fired. That has no effect when comparing revolver to semi-auto pistols. The main determining factor there would be caliber (the actual circumference of the bullet or the diameter of the barrel the bullet travels through). The sound is created by the gunpowder rapidly burning, creating gases to propel the projectile, and what you hear is the gases actually escaping the barrel. The higher the caliber, the bigger the projectile, the more powder that's required to propel it out of the barrel.
I'm sure you've seen a silencer used by an assassin in the movies. Those work by changing and slowing down the way that the actual gasses are allowed to exit the barrel. Even those are illegal in some places and/or require special permits to purchase/own.
Let me know if this helped clear a little confusion if any. Also please feel free to ask any questions or express any concerns you may have about firearms here. I'd be happy to answer questions or discuss this with you, as I'm sure also will others.
My firearms experience started when I was just a child and has been reinforced by my father who is a retired police officer. I also enjoy shooting sports and hunting. I have a permit to carry a concealed weapon which requires a certain amount of class time to even acquire (in my state).