Ubuntu has many "flavours" customized distributions. Regular Ubuntu brings the operating system + Unity (by default), you can install others.
LUbuntu is Ubuntu, but instead of Unity it brigs the desktop environment LXDE:
XUbuntu is also Ubuntu, but brings the desktop environment XFCE:
256 MB of memory. Installing with the Alternative CD requires only 64 MB.
Once installed, it is strongly recommended to have at least 512 MB of memory, but you can run with 256 MB too.
When you install Xubuntu from the Desktop CD, you need 4.4 GB of free space on your hard disk. The Alternate Install CD only requires you to have 2 GB of free space on your hard disk.
This latest distributions, are aimed at people that don't have faster/newer pc's, and so don't have many memory or cpu avaiable.
The're supposed to be used with computers that have low hardware specs.
To install LUbuntu or XUbuntu, you just have to download their ISO's and install it normally like you would on Ubuntu (boot up from CD, USB stick, whatever).
A Pentium II or Celeron system with 128 MB of RAM.
People tend to install this distributions because they requirer few hardware specs, and the cool thing is that you can have an Ubuntu on an older PC.