ownCloud is software for file hosting, synchronization, file sharing and collaborative document editing.
The ownCloud user manual can be found here.
You can access your ownCloud via the web interface as well, at https://cloud.yourdomain.com/owncloud.
Your ownCloud files are also available in the webmail interface (Roundcube), where you can add them as attachments to emails. For this to work, it is recommended to keep the password of your owncloud account the same as your email account.
The media directory
The files in the directory media are accessible by all other ownCloud users on your installation, by PLEX and by Deluge. It is recommended to use this directory only for media you want to stream with PLEX.
ownCloud versus Nextcloud
Nextcloud is a fork of ownCloud that has become more popular in recent years. I decided to use ownCloud because upgrades are smoother in ownCloud, making it easier for system administration.
Nextcloud has recently (August 2020) started offering client-side encryption in its Desktop and mobile app (but not in the browser), which ownCloud does not yet offer. Owncloud desktop users can still locally encrypt their files with Desktop applications such as Cryptomator (see encryption). However, this is not as user-friendly as Nextcloud's builtin solution. If Nextcloud continues to improve on its client-side encryption feature and ownCloud does not develop one I will consider switching to Nextcloud.