Microsoft 365 is a subscription that includes premium apps such as Word, Excel, PowerPoint, OneNote, Outlook, Publisher, and Access (Publisher and Access are only available on PC). The apps can be installed on multiple devices, including PCs, Macs, iPads, iPhones, Android tablets, and Android smartphones. Microsoft 365 also includes services like 1TB of OneDrive storage, 60 Skype call minutes per month, and Microsoft support via chat and phone. With a subscription, you automatically have the latest versions of the apps as updates become available.
With a one-time purchase of Office 2021, you get classic apps like Word, Excel, and PowerPoint for PC or Mac, but without the services included in the Microsoft 365 subscription. No upgrade option is included with a one-time purchase. If you plan to upgrade to the next major version, you'll need to purchase it at full price.