Unity 8 was intended to be the next generation Unity shell based on Qt/QML, targeting convergence across form factors. It is the UI used for Ubuntu Touch phone and tablet devices, and a desktop mode was being developed. Its discontinuation was announced in April 2017.
The Unity 8 shell was intended to be the primary user interface for Ubuntu devices. It is used by default on Ubuntu phone and tablet devices, and was intended to eventually replace Unity 7 on desktop systems.
As of April 2017, all further development on Unity 8 by Canonical has been discontinued, and the new plan is to replace Unity 7 with GNOME Shell as the default desktop shell for Ubuntu by 18.04 LTS. The Ubuntu Insights blog post announcing this decision and its rationale may be read here.