There are two protocols we use when we talk about the consensus protocol of Polkadot, GRANDPA and BABE (Blind Assignment for Blockchain Extension). We talk about both of these because Polkadot uses what is known as hybrid consensus. Hybrid consensus splits up the finality gadget from the block production mechanism.
This is a way of getting the benefits of probabilistic finality (the ability to always produce new blocks) and provable finality (having a universal agreement on the canonical chain with no chance for reversion) in Polkadot. It also avoids the corresponding drawbacks of each mechanism (the chance of unknowingly following the wrong fork in probabilistic finality, and a chance for "stalling" - not being able to produce new blocks - in provable finality). By combining these two mechanisms, Polkadot allows for blocks to be rapidly produced, and the slower finality mechanism to run in a separate process to finalize blocks without risking slower transaction processing or stalling.
