I'm not sure that Apple did a good job. For example, here's part of the explanation:
An update is generally smaller in size than an upgrade. An update is something you can download, unlike a software upgrade. It's wise to install the latest updates available for a software product. You can use an update only if there is a prior version of the software installed on your computer. Mac OS upgrades are sometimes also called "maintenance releases" or "dot releases".
Maybe it didn't clear up the difference between an upgrade and an update, but the document did give me a smile.