Blockchain is a cryptographically secured, public ledger that removes the need for a central authority to maintain the data. It comprises of blocks that are a collection of data. These information filled blocks are chronologically connected to other blocks to form a block-chain. The information on these blocks comprises:
Time, date, and money spent on the purchase
A digital signature that contains information related to the transaction's participants
A unique hash that helps distinguish between blocks
Consensus is essential to ledger technology, and it gives it its decentralization ability. Any update made to the ledger requires validation from the network's peers.
Blockchain Explained section describes this technology in-depth.