Lubuntu is an official Ubuntu flavor. 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. Until and including 18.04 "Bionic Beaver" Lubuntu used the LXDE desktop environment, but releases from 18.10 on use LXQt.
Lubuntu is an official Ubuntu flavor (since 11.10 "Oneiric Ocelot") 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 (until 18.04 "Bionic Beaver) or LXQt (from 18.10 "Cosmic Cuttlefish" on) desktop environment.