Ubuntu made quite a splash when it came on the Free Software scene, and for good reason: Canonical Ltd., the company that created this distribution, started by hiring thirty-odd Debian developers and publicly stating the far-reaching objective of providing a distribution for the general public with a new release twice a year. They also committed to maintaining each version for a year and a half.
These objectives necessarily involve a reduction in scope; Ubuntu focuses on a smaller number of packages than Debian, and relies primarily on the GNOME desktop (although an official Ubuntu derivative, called “Kubuntu”, relies on KDE). Everything is internationalized and made available in a great many languages.
So far, Ubuntu has managed to keep this release rhythm. They also publish Long Term Support (LTS) releases, with a 5-year maintenance promise. As of April 2015, the current LTS version is version 14.04, nicknamed Utopic Unicorn. The last non-LTS version is version 15.04, nicknamed Vivid Vervet. Version numbers describe the release date: 15.04, for example, was released in April 2015.
Ubuntu has reached a wide audience in the general public. Millions of users were impressed by its ease of installation, and the work that went into making the desktop simpler to use.
Ubuntu and Debian used to have a tense relationship; Debian developers who had placed great hopes in Ubuntu contributing directly to Debian were disappointed by the difference between the Canonical marketing, which implied Ubuntu were good citizens in the Free Software world, and the actual practice where they simply made public the changes they applied to Debian packages. Things have been getting better over the years, and Ubuntu has now made it general practice to forward patches to the most appropriate place (although this only applies to external software they package and not to the Ubuntu-specific software such as Mir or Unity).