Lubuntu is a faster, more lightweight and energy saving variant of Ubuntu using LXDE, the Lightweight X11 Desktop Environment. LXDE is a relatively new, but increasingly popular, lightweight desktop environment, similar in impact to XFCE. LXDE is gaining many fans, especially since it was adopted as the default desktop for Knoppix, so an Ubuntu variant based on LXDE was quite inevitable.
This distro combines all the positives of Ubuntu; the ease of use, fast installation, good documentation and strong community support, with the fast, responsive LXDE desktop. For all its plus points, Ubuntu's GNOME desktop is just too much for smaller and older hardware, it can really hurt the performance of a netbook. Until now, the only choices have been Xubuntu with XFCE, unless you want the non-standard interface of Ubuntu Netbook Remix. Lubuntu fills a real need and is a combination of the most popular distro with one of the best lightweight desktops out there.
If you have a netbook or an older computer, this has got to be on your shortlist of distros to try.