Updating a site and upgrading a site may seem similar but have very different meanings. When we use the term update your site and keeping it up-to-date, we are referring to keeping aware of when the modules and core of your site. This means applying the changes as they occur from 7.x to 7.x. Whereas when we refer to an upgrade your site, it refers to your site core going from 7.x to a 8.x version.
Update - Updating a site is usually in reference to minor version changes.
Upgrade - Upgrading a site is usually in reference to major version changes.
Upgrading usually takes a long time and lots of testing to verify the upgrade went smoothly. While updating your site may take few hours or less but happens much more frequently. Good thing upgrades are needed less often. Usually upgrading your site is only necessary when the core version of your site is no longer maintained because it is so outdated.