A block is data structure that stores transaction data during a given time period. Blocks are the foundational parts of a blockchain. Every block contains a reference to the preceding block and the one after it in the form of a hash (an alphanumeric code). Blocks are ordered sequentially. Forging the ledger without hacking a full blockchain is nearly impossible.
In addition to a timestamp, transaction record, and the hash of the previous block, every block contains a unique cryptographic puzzle. In order to add a new block to the chain, this puzzle needs to be solved in a process called mining. Those who helped solve this problem are rewarded, and so, in the case of the Bitcoin blockchain, every block also includes details of the addresses that have earned that reward.
The block structure makes blockchains extremely difficult to hack or falsify. Additionally, the decentralized nature of blockchain makes it even more difficult to attack as it is located on the local computer of every one of the blockchain’s users.