Bitcoin is dependent on the blockchain that underlies and structures the system. The blockchain is the vertebrae of the protocol and the glue that holds the network together. It is simply a vast, distributed public ledger of account. It keeps track of every transaction ever made in the network, and all transactions are timestamped and verified by network miners. This is how it works: miners with specialized computers compete to solve mathematical puzzles with other computers, and once they solve a puzzle they are awarded with some Bitcoin, but they also add a “block” of completed transactions to the blockchain for future viewing and verifiability. Once a block is added to the chain the cycle repeats itself, and the computers continue to compete to solve these difficult problems. Every transaction on the blockchain is completely transparent and accounted for in its log. Anyone can see the public keys of any transaction they want (although there are no names associated with transactions). One could go all the way back and view the very first transactions ever made on the first block ever created. This block was unironically called the Genesis Block.