As the work on Nakamoto has progressed, there have been different versions of testnet, often referred to by their noble gas names like Neon, Argon, etc. However, those distinctions have been more geared toward sprints and projects the core devs have broken down work into. You can still call them that if you like, but going forward, it is simpler to look at the two testnets based on what they offer.
The two testnets are: