How it works
One or more Nextcloud accounts are created on our server, possibly with shared directories, contacts and/or calendars. The Nextcloud client is then installed on your computer(s), tablets or phones. It can also be accessed without the client via webbrowser, however, for mobile devices, the app is preferred by most of our users.
On initial set up, one or more directories (folders) on your workstation are designated to be synchronized by the Nextcloud client. When configured, files in those (possibly empty) folders are compared to all files on the servers (possibly empty) folders and copies made so the same directories and files exist on both machines. Files with the same name on both have their dates and sizes compared. If these match, they are left alone. If they do not match, the newest file is downloaded/uploaded, overwriting the older file.
At this point, you can begin working on the files on your computer. As you make changes, the Nextcloud client notices and immediately uploads the modifications. If you are not online, the Nextcloud client waits until a connection is available, then the synchronization begins. (If you’ve used Dropbox or OneDrive apps, this is similar to how they work.)
You can also use the Web Interface to manipulate files directly on the server. Any changes made are then synchronized to your computer.
Nextcloud is scalable. You can connect multiple devices to the same account. When a change is detected, it is uploaded to the server. Any other connected devices then download the changes to their local folders. The other device can be another computer (with a different operating system if you want), a tablet or a smartphone. Now, when changes are made on one any of the devices (or on the server), those changes propagate to all the other devices which share that account.
Note: If, for some reason, two devices edit the same file at the same time, it is very possible for the latest one to overwrite the newer one. Nextcloud detects such conditions and saves one of the copies with a special file name ending, and manual editing to merge the changes is required.
Due to smaller drive sizes, tablets and phone have remote access to the files, but opening one requires downloading it to your device, then uploading back to the server if changes are made (contacts and calendars are fully synchronized, however).
For some practical, real world scenarios of using Nextcloud to increase your productivity, see Nextcloud Usage Common Scenarios.