It's still uses the Debian Root of Linux, but you preconfigure it with those modules you want and it will only download those kernel routines you need and nothing more. It saves RAM, SD and CPU resources while increasing the speed of the OS without the Bloat-ware that slows things down.
 
It is for several boards including the Raspberry Pi, Banana Pi and Orange Pi.
 
Check it out at: http://dietpi.com

Reading further, according to them, Text version of Raspberry Pi Jessie "Lite" Linux is about 1GB, which sounds about right. But their version is about 400MB; In looking at their installation options that is where things become interesting. Jessie Lite has everything in its place to load up when needed, DietPi does not have those options - it is just the OS.
 
For example, Jessie (and Jessie Lite) has the GPIO Library to gain access and use the GPIO through programming languages like Python and Scratch, which are also installed on the OS. DietPi does not have the GPIO Library installed and has to be put in as a requested option during installation. The Languages themselves also have to be installed as requested options as well. Without the GPIO Library and the programming languages to run them, one is saving almost 200mb of space. But one also has no access to the GPIO ether.
 
One thing I might agree on is the LAMP Web Server Stack being added as an option. In the past, LAMP was an installaible option Under Wheezy and older version of Jessie. I do not know if it is still an installable option under the Jesie/Pixel OS or it if it is installed "without permission." In most cases, the Raspberry Pi does not need to be a Webserver, and thus does not need LAMP installed. Not having it saves 200MB to nearly 1GB of space depending on the options one chooses for their webserver (like TomCat and other Java / D-Base options).
 
Though DeitPi makes it easier to trim down and build a slim system, it is no easy task. In the least it should not be left off to the beginner. Raspberry Pi OS it still the better option for one to learn from. Once they get brave to try other boards, I would still say stick with that system's choice of OS or at least use Armbian as is the case with the Orange Pi. Once one learns the intricacies of Linux and their board, then move onto Diet Pi.

That it is. In many fields like I like to point out - robotics, after all the research and debugging is done, one can but the program into a minimalist OS such as this, test it out, fine tune it, and let it run on its own with 1/2 the resources as the prototype did on Jessie Raspbian.
 
Somewhere I have an old copy of Kiosk Linux. It basically makes any PC into a Kiosk system through linked webpages either internal or external to the system. That would be nice to port over to a tiny Pi system and build Kiosk panels from large screen monitors.