Visio Online is a browser-based version of Microsoft Visio, Office’s vector diagramming app. Visio Online is a "lite" version of the full-fledged desktop app and provides fast opening times for viewing .vsdx and .vsd files, and editing of .vsdx files. Visio Online is great for viewing Visio files and making simple edits from basically any browser on almost any device. Viewing Visio files is included in all Office 365 subscriptions, but the editing feature requires the purchase of an extra subscription.
A number of diagram types, with specific shapes, are provided with Visio Online, and a large number of other shapes can be downloaded from the web. Most Visio files will open directly in Visio Online when using SharePoint Online, OneDrive for Business, MS Teams, Yammer, and some other Office 365 apps. Files open initially in reading mode, but to edit the files, simply click the "Open Diagram" button and choose to "Edit in Visio", for the desktop, or "Edit in browser" to use Visio Online directly in the browser.
Functionality in Visio Online is generally limited to day-to-day editing, text entry, connecting shapes, some formatting, and comments. All changes made in Visio Online are saved automatically; there is no save button.
You cannot edit shape data or hyperlinks in Visio Online, or use any of the add-ons provided in the desktop edition. Macros will also not run in Visio Online and protected documents cannot be opened.
If Visio Online doesn't provide the features you need to do the job, simply click the "Edit in Visio" button in the menu bar to access the desktop version of the app and its more complex features. Visio Pro for Office 365 is compatible with Office 365 and is frequently updated. However, owners of Visio Standard or Professional 2016 can enable them to be compatible.
- David Parker, Visio MVP