What is Casper Friendly Finality Gadget (CFFG)?

Casper Friendly Finality Gadget (CFFG) is the hybrid Proof-of-Work (PoW) and Proof-of-Stake (PoS) protocol that enables Ethereum to transition to a PoS system. Through the CFFG, Ethereum's blocks will still be mined using PoW, and only ~2% of blocks will be finalized by network validators using this system. CFFG is part of Ethereum's multi-step transition to a full PoS system, and Ethereum's final version will likely run on something more akin to Casper Correct by Construction (CBC).