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 names, without comment, the bank bailouts 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 the block awards can never be spent, because of a quirk in the code. Every later block descends from this one, whose previous-hash field is zero.