Dumb question

New to stocks
I have a stock that has gone up a lot

I want to protect myself

Let’s say the stock is $100 and I want to sell it if it goes to 80

How do I automatically do that ?

I’m with Charles Schwab

I see my sell options as

Limit
Stop
Stop limit
Trailing stop

What’s the difference in stop and stop limit ?
Which option do I choose ?

Thanks