A public blockchain is a decentralized and open-source blockchain network that allows anyone to participate, validate transactions, and access data. Public blockchains like Bitcoin and Ethereum are transparent and trustless, ensuring that all participants have equal access to the network. They are highly secure due to their consensus mechanisms (such as Proof of Work or Proof of Stake), which validate transactions and prevent fraud. Public blockchains are ideal for applications like cryptocurrency transactions, decentralized finance (DeFi), and other decentralized applications (DApps).