Office 365 is a subscription that includes the premium versions of the Office apps across all your devices, monthly exclusive feature updates, and 1TB of cloud storage. Office 2019 is a one-time purchase that includes classic versions of the Office apps installed on one PC or Mac (or 5+ with a volume license).
Office 365 subscription plans
Create your best work with Office 365. With Office 365 subscription plans you get premium versions of Office applications: Word, Excel, PowerPoint, Outlook, Publisher, and Access as well as access to additional OneNote features (features vary) (Publisher and Access are available on PC only.) Office 365 works across multiple devices and includes 1TB of online storage per person with OneDrive. Apps are always updated, so they're never outdated.