Pairadocs,
Let me put my two cents in.
"Shock". This an action, not an additive. You couldn't know that as a newbie, but pool chem companies write that on labels so that lots and lots of pool owners think it's a thing, not an action. You are not alone in this. We are battling their efforts to keep you in a constant state of terror if you don't regularly add lots of exotic-sounding and expensive chemicals.

The bottom line is you don't need most of them, even when you are battling a horrific algae bloom that's turned your pool into a toxic science experiment.

To shock a pool (I don't really like the term at all) is simply to raise your sanitation well above normal maintenance levels to kill anything and everthing that could be growing. There's lots of things you can add to to this, good, bad, and terrible. That's why we suggest you always check what they are calling "shock" before you add it.

Another trick pool chem companies do is simply package a pre-measured dose of a chemical into a bag and call it "shock". Most often, it's nothing more than an expensive way to buy di-chlor or cal-hypo powder that you can buy in bulk.

Here is what I've seen called shock--other folks feel free to chime in!
Di-chlor shock. Same as Di-chlor in a bucket. Adds chlorine nicely but also adds CYA and lowers pH. However, it's not as drastic to CYA or pH as Tri-chlor pucks. Unfortunately, you may find some coming marked "double acting", meaning they are loaded with copper. Don't use this, ever.

Cal-Hypo shock. This is cal-hypo powder. Sometimes it's 68% chlorine--the good stuff at better pool stores, or it's 48% chlorine--the cheap garbage that HTH is pushing on the discount houses. Adds calcium AND can temporarily make your water milky. If your calcium level is elevated, it can cause scaling or milk-colored pool water.

Liquid Shock: USUALLY, this is just liquid chlorine--ie, bleach. Sometimes it's 6%--which is just Ultra bleach. Sometimes it's 12 or 12.5% listed, but has probably deteriorated to 10% or less unless it's only been off the truck a day or two. OCCASIONALLY it is more economical to use this instead of bleach (then I buy it). Usually, it's not.

Sometimes Liquid Shock is NOT chlorine--using this in a chlorine pool can be SERIOUSLY problematic. I believe this is used only in bromine or bacquacil pools. Never, EVER add Liquid Shock if it isn't Sodium Hypochlorite--bleach--and nothing else.

Lithium Chloride. This is a new non-calcium, non-stabilized chlorine. It's absurdly expensive, is less than 50% active chlorine, and, is probably adding lithium to your water--a metal. I have NO idea the effect lithium metals have on pool water, but generally, only metals like calcium and sodium are OK. (we forget that Ca and Na are metals because we never see them that way--but they are greyish metals).

I use bleach to shock. Occasionally, I'll use the 12% liquid chlorine/bleach. Even more occasionally (like when I have no CYA and need some) I'll use di-chlor powder.

So, please, before you add something, if you don't know what is in it, ask us first, and we'll be happy to say "Sure, go ahead" or "NO! STOP! Don't add that $#!$!"