PET (Puppy's Extra Treats) is the unified package system adopted in Puppy version 2.14 and later. They are used to easily add extra software to Puppy. They can be accessed from repositories by the Puppy Package Management system. Alternatively you can run a Pet file and it will automatically install the software.
Pets are tarballs, just like '.tar.gz' files, with a MD5sum appended on the end. The in-built MD5sum is handy, as the file can be checked for integrity after being downloaded. They are used for packaging programs designed or compiled for Puppy and provide desktop icons, menu info etc specific to Puppy. Tarball files have a .tar extension. Or tar.gz if compressed with gzip. Pet files on the other hand have a .pet extension.