A block is the fundamental unit of record-keeping. If you imagine a blockchain as a digital “Book of Truth” that everyone in the world can see but no one can erase, then a block is a single, completed page in that book. Once a page is full, it is “bound” to the book permanently, and a new page is started.
The concept was introduced in 2008 by the mysterious Satoshi Nakamoto. Before blocks came into being, digital money had a “double-spending” problem. This made it easy to copy and paste a digital dollar like a JPEG. Nakamoto’s brilliant idea involved grouping transactions into blocks and connecting them with a mathematical seal. The result ensured that once a transaction is “written on the page,” it can’t be spent again. If a hacker tries to change even one tiny comma in an old block, that block’s fingerprint changes instantly.
Every block consists of two main parts, the header and the body. The header acts as the block’s metadata. It includes a timestamp and a unique digital fingerprint called a Hash. Most importantly, it contains the fingerprint of the previous block. The “chain” in blockchain comes from how blocks interact. Each new block includes the hash of the block that came before it. If a single character of data is changed in an old block, its hash changes, which breaks the connection to every subsequent block. Remember, each new block’s header contains the “DNA” (the hash) of the one before it, they form an unbreakable sequence.
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