Difference between "Domain" and "Siteurl" in site settings?

I have my muti-site WordPress set as a sub-domain type. Then I installed the domain mapping plugin. However, I noticed that the sub-domain is sometimes used. Can I change the Siteurl in the site settings to my intended http://custom-domain1.com? I think I have seen that changing the “Domain” is what would mess things up. Can anyone shed light on this please?