Lubuntu

Lubuntu is the LXDE and LXQt version of Ubuntu, and is considered the lightest Ubuntu distro. It is optimized for old computers, and is surprisingly stable. Lubuntu is targeted at "normal" PC and laptop users running on low-spec hardware. Such users may not know how to use command line tools, and in most cases they just don't have enough resources for all the bells and whistles of the "full-featured" mainstream distributions.

Lubuntu vs. Ubuntu
Lighter applications than the usual versions of Ubuntu are included. LXDE and LXQt are extremely light, and Lubuntu is considered a light-weight distribution.

Features
Lubuntu releases up to 18.04 feature the LXDE desktop environment, and releases since 18.10 feature LXQt, along with an LXDE application set.

Lubuntu uses Openbox as its window manager.

LXQt
The LXDE developers are putting their focus on a new project called LXQt, the result of a merge between RazorQT and a Qt port of LXDE. The Lubuntu team has expressed interest in supporting LXQt, and has replaced LXDE with LXQt in 18.10.