The EOS blockchain uses a Delegated Proof of Stake (DPoS) consensus mechanism with 21 block validators and integrated Byzantine fault tolerance. Delegated Proof of Stake is a consensus mechanism where blocks are validated by a pre-selected group of nodes and it allows for high transaction throughput.
Byzantine fault tolerance is the ability of a network to handle situations where nodes go down or malicious nodes broadcast faulty information. EOS is theoretically Byzantine fault tolerant because 15 out of the 21 block producers are required to confirm a transaction (a 2/3 majority).
For a more detailed discussion on Byzantine fault tolerance and the reason a 2/3 majority is important, refer to our Practical Byzantine Fault Tolerance article.
EOS transactions are typically confirmed within 1 second with a 99.9% certainty, as a new block is created every 0.5 seconds. Dan Larimer stated in an April 2018 blog post that EOS can theoretically support over 1,000 transactions per second and aims to scale to 6-8,000 transactions per second in the future.
EOS also implements a mechanism called Transaction as Proof of Stake (TaPoS), where every transaction must include part of the hash of a recent block header. TaPoS makes it difficult to forge counterfeit chains considering the counterfeit chain would not be able to migrate transactions from the legitimate chain.