What is the difference between a permissionless and a permissioned blockchain?

Permissionless = anyone can join as reader or writer with no gatekeeper (Bitcoin, Ethereum); permissioned = a central authority decides who may read or write (Hyperledger Fabric, R3 Corda).

• Permissionless blockchains are open and decentralized. Any peer joins or leaves as reader/writer at any time; no central entity manages membership or can ban anyone. Because it's open, written content is readable by any peer — though cryptography can still hide privacy-relevant data on top (e.g. Zerocash).
• Permissioned blockchains restrict participation. A central entity grants individual peers the right to write and/or read. This makes them look a lot like a centralized database — which is exactly why the paper asks whether they're worth it over one.

The key axis is who controls the writer set: nobody (permissionless) vs a gatekeeper (permissioned).

Go deeper:

• Permissioned blockchain (Wikipedia) — the access-control layer that gates who may read or write.
• Blockchain (Wikipedia) — the open, permissionless model (Bitcoin, Ethereum) for contrast.