Lubuntu
Lubuntu is one of several officially recognized flavors of Ubuntu. An Ubuntu flavor is fully integrated in the Ubuntu project and contributes significantly and directly towards the project. What separates Lubuntu from Ubuntu is LXDE, a clean and easy-to-use desktop environment with minimal hardware requirements. While the hardware requirements of Ubuntu are far from steep, it's not hard to notice that the distribution doesn't run quite as well on older machines as it does on new computers. Lubuntu, on the other hand, runs without a hitch even on computers with just 1 GB of RAM and a Pentium II or Celeron CPU.
Kubuntu
Ubuntu comes with the GNOME desktop environment, which strives to be elegant, easy to use, and, most importantly, stay out of the user's way. To accomplish these goals, GNOME developers carefully choose which features to include and which not, often leaning on the side of minimalism. Because GNOME's approach isn't for everyone, many Ubuntu fans install Kubuntu instead. With Kubuntu, you can enjoy KDE and the fabulous Plasma desktop. KDE is characterized by its endless customizability, a wide variety of applications, and a strong focus on finding innovative solutions to old and new problems alike.
Linux Mint
Ubuntu is known for its user-friendly nature, but it's also firmly committed to open source software and, as a consequence, doesn't include many proprietary and non-free codecs. The members of the team behind Linux Mint understand that regular users don't want to spend hours tweaking their operating system after installation just to get a video to play. They want something that's both powerful and easy to use, and that's exactly what Linux Mint is. Right out of the box, this Ubuntu derivative comes with full multimedia support and provides around 30,000 packages. It's also available in several versions, allowing users to pick their favorite desktop environment with a single click.
gNewSense
In a way, gNewSense is the exact opposite of Linux Mint. Whereas Linux Mint builds on Ubuntu by adding proprietary and non-free codecs to make it more user friendly, gNewSense builds on Ubuntu by stripping it from its non-free components while being committed to the philosophy of the Free Software Movement. Before he switched to Trisquel GNU/Linux, Richard Matthew Stallman, an American free software movement activist and programmer often known by his initials, rms, used gNewSense on his Thinkpad T400s computer together with free initialization program libreboot.
Ubuntu Studio
Ubuntu Studio is aimed at creative people who want an easy access to open source audio, graphics, video, photo, and publishing software. It's perfect for those who would like to try what open source software can offer to content creators, but it's also used by established artists, who appreciate its convenient nature and optimization.