Microsoft 365 Online is a free version of Office that you can use in a web browser. You can do almost everything with the Office Online apps, but some more advanced features are only available with the Microsoft 365 or Office 2019 desktop apps.
Microsoft 365 is a subscription service that ensures you always have the most up-to-date Office apps from Microsoft. With Microsoft 365 Family, you can install Microsoft 365 on all your devices and sign in to five at the same time―that means you can use Office no matter where you are or what device you’re on. This includes PCs, Macs, tablets, and phones. You can also share you subscription with up to five other people.
Office 2021 is sold as a one-time purchase (without a subscription), which means you pay a single, up-front cost to get Office apps for one computer (PC or Mac). One-time purchases also will not receive any new features and don’t have an upgrade option, which means if you plan to upgrade to the next major release, you'll have to buy it again.
