Glossary
ZK-SNARK
A succinct, non-interactive zero-knowledge proof — small in size and fast to verify, but typically requires a trusted setup to generate proving keys.
SNARK = Succinct Non-interactive ARgument of Knowledge. Proofs are on the order of kilobytes and verify in milliseconds, which is why ZK rollups can post one to Ethereum on every batch.
The catch is the trusted setup: parameter generation requires at least one honest participant in a multi-party ceremony, or else proofs can be forged. Modern "universal" SNARKs (PLONK, Halo2) reduce the setup to a one-time, application-independent ceremony.