Lubuntu is a fast and lightweight operating system developed by a community of Free and Open Source enthusiasts. The core of the system is based on Linux and Ubuntu . Lubuntu uses the minimal desktop LXDE, and a selection of light applications focusing on speed and energy-efficiency. Because of this, Lubuntu has very low hardware requirements.
Lubuntu is a flavor of Ubuntu based on the Lightweight X11 Desktop Environment (LXDE), as its default GUI. The goal is to provide a very lightweight distribution, with all the advantages of the Ubuntu world (repositories, support, etc.). 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.
With many LXDE components, Lubuntu also uses well-known applications, such as Firefox, Openbox, Pidgin, to name a few. The Lubuntu project wiki contains more information on the project and the applications used available.
Based on the lightweight LXDE desktop environment.
Pcmanfm, a fast and lightweight files manager using gio/gvfs.
Openbox, the fast and extensible, default windows-manager of LXDE.
Lightdm, using the simple GTK greeter
Firefox, the famous web browser.
Based on Ubuntu 14.04 LTS.
A Pentium II or Celeron system with 128 MB of RAM is probably a bottom-line configuration that may yield slow yet usable system with a standard lubuntu desktop.
14.04 32 bit ISO require your CPU to have Physical Address Extensions, or PAE. "PAE is provided by Intel Pentium Pro and above CPUs, including all later Pentium-series processors (except most 400 MHz-bus versions of the Pentium M)." - If you have an error with Celeron M reporting "NON-PAE CPU" and would like to install Lubuntu 14.04, please see this page
For PowerPC, it is known to run on a G4 running at 867MHz with 640MB RAM.
For Intel based Macs, lubuntu should run on all models.