PKPass is the file format Apple Wallet uses for passes. A .pkpass file is a zip archive containing pass.json, which defines the pass's type, fields, colors and barcodes, the images used in the layout, optional translations, and a manifest with a cryptographic signature.
The signature is what makes a pass trustworthy. Every pass is signed with a certificate tied to a pass type identifier registered in an Apple Developer account, plus Apple's WWDR intermediate certificate. iPhones verify the signature before adding the pass, so a pass cannot be forged or silently altered.
The name is easy to confuse: Apple's developer framework for working with passes is called PassKit, one wallet platform vendor is called PassKit, and the file format is PKPass. In practice, people say "a pkpass" for the file itself.
A PKPass file can be delivered almost anywhere: attached to an email, downloaded from a link, presented at the end of an enrollment flow, or returned by an API. Once added, the pass stays connected to its issuing platform through device registration, which is what allows balances, tiers and offers to update on the pass after it is in the wallet.
Google Wallet does not use PKPass. Android passes are created as objects through the Google Wallet API and delivered with a save link, which is why a wallet platform issues both formats from a single pass definition.