--- From apt show owncloud-client:

Description: folder synchronization with an ownCloud server - GUI

• The ownCloudSync system lets you always have your latest files wherever you are. Just specify one or more folders on the local machine to and a server to synchronize to. You can configure more computers to synchronize to the same server and any change to the files on one computer will silently and reliably flow across to every other.
• owncloud-client provides the graphical client specialising in synchronizing with cloud storage provided by ownCloud.

Installing Owncloud server in Debian 9

wget -nv https://download.owncloud.org/download/repositories/10.0/Debian_9.0/Release.key -O Release.key
sudo apt-key add - < Release.key
sudo nano /etc/apt/sources.list.d/owncloud.list

add this line

deb http://download.owncloud.org/download/repositories/stable/Debian_9.0/ / 

and then:

sudo apt-get update
sudo apt-get install owncloud-files