This document summarizes blockchains and consensus algorithms. It discusses public blockchains like Bitcoin which use proof of work to sequence transactions in time-stamped blocks. Private blockchains are permissioned and shared, representing public asset ownership through cryptographic keys. They provide better immutability than traditional databases without central authorities. Benefits include reduced costs through resource pooling and real-time settlement between counterparties. Public blockchains enable global, permissionless use and minimize trust through censorship resistance. Potential issues include liquidity risk, increased attacks, and legal/operational challenges. Proof of stake uses voting while proof of work aligns incentives and reduces dishonest actors as the token value increases.