The Ubuntu GNU/Linux distribution is produced by Canonical, a company founded and led by Mark Shuttleworth, an entrepreneur and former Debian developer.
Mir was announced back in March by Ries, because, according to him, "none of the existing solutions would allow us to implement our vision without taking major compromises which would come at the cost of user experience and quality."
Most of the Linux world is looking to move from the venerable X.org display server to Wayland which is in development. For a while it looked as though Canonical would also go with the pack - until the announcement in March.
The migration path Ries laid out for the transition to Mir is given below:
Ubuntu 13.10: XMir on Mir by default, with a fallback session to X where there is no Mir driver support, supported for nine months.
Ubuntu 14.04 LTS: XMir as default with the fallback session removed, full Mir driver support, traditional LTS support for five years
Ubuntu 14.10 and beyond: Mir stack as default, including rootless X support for legacy X applications, supported for nine months.
(XMir: X & Unity7 running on top of the system compositor Mir. Mir stack: Mir as system compositor with Unity 8 as session shell on top.)
Along with the announcement, Canonical has also released a video of various desktop environments running on XMir on Mir on Ubuntu.