The breakdown as I understand is as follows:

- Owncloud: The 'original' platform, written in php, still developed today

- Nextcloud: A fork of Owncloud (php) by some of the people that worked on it, including the project founder over directional differences, now more widely used than the original Owncloud

- Owncloud infinite scale (OCIS): An implementation of the Owncloud server in Go, with the goal of making it faster and more scalable than the PHP version

- Opencloud: A fork of Owncloud infinite scale (Go) after an acquisition of ownCloud, the company.