From the wikka:
I think it's time to update this section to highlight the fact that Puppy 4.XX series only has 3 compressed files, and zdrvxxx.sfs has been incorporated into pupxxx.sfs as well.Open the Live CD and look at the contents. You will see that the complete operating system is contained in just four compressed files. They are vmlinuz, initrd.gz, pupxxx.sfs and zdrvxxx.sfs. All you need to know for now is that the first two files contain the code that Puppy uses to get itself started. The third file contains the application software like word processors and web browsers. The fourth file contains hardware drivers.
Don C.