Basic definition question: Cooktops

Hello again,

I took the Wedgewood stove out of my Dolphin and scrapped it. I am
interested in replacing it with a cooktop (I'm not much into
baking...;-D...).

There are plenty of options at various RV stores, but I'm a bit
confused...

What is the difference between a "drop-in" cooktop and a "slide-in"
cooktop?

Given that the hole in my counter is about 20" X 20" looking from
above and 20" X 16" looking from the front, I don't quite see how it
would matter which way the cooktop goes into the hole.

Thanks for your help.

Buzz L.
Currently in Mesa, AZ