How it works
Users sign transactions with their private keys and broadcast them to the network. Nodes check that each transaction is valid, and a consensus process such as proof of work or proof of stake decides which block is added next. Once a block is deep in the chain, reversing it becomes practically impossible.
Because anyone can run a node and read the ledger, nobody needs to trust a single company to keep the books.
Where it is used
Bitcoin uses its blockchain for payments. Ethereum, Solana and similar networks also run smart contracts, which power stablecoins, decentralized exchanges, lending apps and NFTs. Companies use private blockchains for settlement and supply chains, though these are controlled by a known group of members.