No, it’s not a chain that stretches around your block. It is a peer-to-peer managed, publicly readable and secure database, chunks of which are located in multiple places. Each « place » (or node) is an isolated and independent computer on a network. Each computer is equipped with special software for management of the blockchain. The blockchain database itself contains an ever-expanding list of verified transaction data. Think of a blockchain as a bullet-proof record of proven transactions that everybody (with the appropriate software) can check.