PET stands for "Puppy's Extra Treats" and is a unified package system used by Puppy Linux 2.14 and later. PET files can be opened by the included PETget utility, or with any TGZ file extractor. When extracted, the package's directory structure is projected onto the root level of the Linux file system.
PET packages are commonly referred to as "pets." They replace .PUP files used by earlier versions of Puppy Linux. Older PUP files can be converted to PET files using the pup2pet program included with the "PET tools."