The Bitcoin Genesis block is the first block in the blockchain ever.
All crypto currencies have a genesis block but Bitcoin’s is that little bit more special as it was the first of the first.
Every block created since the genesis block can automatically trace it as its ancestor by the linkages to previous blocks in the block header.
The following is the block header hash for the first block!
The Genesis block contains a hidden message in the coinbase transaction – it says “"The Times 03/Jan/2009 Chancellor on brink of second bailout for banks.". This is clearly a note from the eponymous Satoshi Nakomoto performing two functions – firstly marking the time of the first block and secondly economically pointing out the problems with the present Government bailouts of too big to fail banks and the moral hazard problem.