What happens during a cPanel Update?

I'm fairly new to cPanel/WHM/Linux. I've learned a lot these past several months. One thing that I have not been part of is updating cPanel. I noticed that if you click on the cPanel version in the upper part of the WHM screen it takes you to the Upgrade to Latest Version page where you can click on the Click to Upgrade button. What happens during an upgrade? Are there any disruptions to web sites and/or email flow? Does the system reboot? Just curious. Thank you.