Yes, this is yet another Linux distribution. What's different here is that Puppy is extraordinarily small, yet quite fully featured. Puppy boots into a ramdisk, and that's it, the whole caboodle runs in RAM. Unlike live CD distributions that have to keep pulling stuff off the CD, Puppy in his entirety loads into RAM. This means that most applications start in the blink of an eye and respond to user input instantly.
What does Puppy boot off?
Anything! My "first take" with installing Puppy was on a tiny USB flash card, as I really like the idea of having all my personal data and the operating system in something that I can carry in a shirt pocket or wallet.
I am gradually testing Puppy booting off different media, and Puppy gets a unique name for each. flash-Puppy boots off a flash card (or any USB storage media) and live-Puppy is a live-CD, that is, Puppy boots directly from the CD just like Knoppix (with the most important difference that once Puppy has booted the CD drive is free for other uses!).