Onsen/温泉 are hot spring baths.
Sento/銭湯 are public bathing facilities.
The big difference is than onsen are natural hot springs, but sento use artificially heated tap water.
They are both awesome at reliving stress.
Onsen are a popular holiday destination.
Up in the mountains, natural outdoor hot spring baths are amazing.
Usually they are separated by gender and you bathe fully nude.
You shower thoroughly using soaps and shampoo.
Only after you’re fully clean and rinsed, you can enter the shared bath.