Lubuntu is a lightweight distribution based on Ubuntu. It and the default applications included use minimal resources. As of the 11.10 "Oneiric Ocelot" release, Lubuntu is an official Ubuntu derivative. Lubuntu uses the Openbox window manager.
Lubuntu is an official Ubuntu derivative known for using less resources and, for the most part, the default applications are "lighter" than those found in Ubuntu. It uses the Openbox window manager and the LXDE desktop environment.
The release notes for the latest release, Lubuntu 14.10, are here.
The release notes for Lubuntu 14.04 LTS are here.