Excerpt. The complete work is at the source.
The first block of the Bitcoin chain was mined on 3 January 2009 and its construction is written into the source of the first release, so that every node can rebuild it and agree on it. The coinbase transaction carries a string: that day's headline from The Times of London.
The headline does two jobs. It proves the block could not have been made before that morning, and it states, without comment, what the system was built in reaction to. The nonce is the number that made the block's hash fall below the target; it was found by trial. The 50 coins it awards can never be spent, through a quirk of the code. Everything the chain has become, from the ledger to the arguments about it, descends from a block whose previous-hash field is zero and whose message is a newspaper.