How to switch between visible buffers in emacs?

If I am in split-screen viewing 2 different buffers on Emacs and the cursor is on the top buffer, what's a quick way to move the cursor to the bottom buffer?

Bonus question: if I know a command, is there an easy way to identify what key-combo it's bound to, if any?

To switch to other buffer use: C-x o.

Describe key: C-h k.

Here is a better solution when you open more than two windows(buffers) in one frame:

(global-set-key (kbd "C-x <up>") 'windmove-up)
(global-set-key (kbd "C-x <down>") 'windmove-down)
(global-set-key (kbd "C-x <left>") 'windmove-left)
(global-set-key (kbd "C-x <right>") 'windmove-right)

Now, you can use C-x UP/DOWN/LEFT/RIGHT to go to the above/nether/left/right buffer when you have three or more in one frame, they are more precise than 'other-window and you don't need to install any package.

You even CAN make it to cycle the buffers in the direction(vertically/horizontally) with one of the above shortkeys with configuration in .emacs/init.el file, but I don't recommend it(besides I don't remember it anymore, you can google it if you want).

Of course, you can use other shortkeys other than the ones I use in my .emacs.