Lubuntu is a lightweight GNU/Linux distribution built on top of Ubuntu. It integrates lightweight and less resource hungry LXDE desktop environment with solid Ubuntu base.
Lubuntu mainly targets old computers and netbooks where resources less comparing to modern laptops and desktop computers.
Currently, Lubuntu makes use of LXDE, in near future, LXDE may be replaced with LXQt, a Qt based implementation of LXDE. Lubuntu also provides an experimental release called Lubuntu-Next to test LXQt desktop.