On-ear vs over-ear headphones: What are the biggest differences?

Given the similar sounding names and very similar designs of on-ear and over-ear headphones, you might be wondering what the difference is between the two. Both of them go over your head and sit on your ears to put audio into them, how could they be that different?

Source : https://www.pocket-lint.com/biggest-differences-between-on-ear-and-over-ear-headphones/