What happens if (or could) a sitting POTUS just quit?

I was reading this thread: http://www.centerforinquiry.net/forums/viewthread/19249/ and the thought occurred to me.
Similarly… Can a POTUS be ‘relieved’ of command? (Probably too much Star Trek watching on my part.)

As for just quitting, sure.
https://www.youtube.com/watch?v=3_DrUaJEBtE
https://www.youtube.com/watch?v=XTFsoElX-a0

This explains pretty well how the 25th Amendment works] Bones used something similar on Kirk once or twice. I was really disappointed when they didn’t do it on day one.

Yes and yes.

Of course he can quit. But our so called president never would. He’s in love with the power to destroy.

Of course he can quit. But our so called president never would. He's in love with the power to destroy.
That's because he has more psychopathic traits than even Hitler had. http://www.ox.ac.uk/news/2016-08-23-presidential-candidates-may-be-psychopaths-–-could-be-good-thing