Lubuntu vs Ubuntu
Ubuntu is a Linux distribution which strives to be easy-to-use regardless of the user's level of experience with computers. It is the most popular Linux distribution and thus a large variety of software has been written for it and a wide variety of laptop and desktop hardware is compatible with it.
Lubuntu is an Ubuntu-based distribution intended for computers with less memory, processing speed, and hard drive space. It uses the "Lightweight X11 Desktop Environment" (LXDE) as an alternative to GNOME, the default desktop environment for Ubuntu.
Both Ubuntu and Lubuntu are designed to boot up quickly. The applications that are included with each are different, though. The word processor, spreadsheet program, and web browser included with Lubuntu all are capable of starting quickly and functioning well with limited RAM. Ubuntu's default programs for these tasks start and run more slowly.
Lubuntu's default applications are both lighter in terms of system requirements, and include fewer features. Lubuntu doesn't include a program equivalent to LibreOffice Impress (presentations) or LibreOffice Draw (publications / layout).
Many desktop-computing programs for Linux are written to integrate well with GNOME, which is installed by default on Ubuntu. This provides a great degree of compatibility for an Ubuntu system. Many, but not all, applications will work as desired in Lubuntu's LXDE too. Installing GNOME to a Lubuntu system in order to increase compatibility is possible but negates most of the advantages Lubuntu would have had over Ubuntu in the first place.