The genesis block is the first block of a blockchain, also known as Block 0 or Block 1. It is hardcoded into the software and serves as the foundation from which all subsequent blocks are built.
Bitcoin genesis block was mined by Satoshi Nakamoto on January 3, 2009. It contains the message "The Times 03/Jan/2009 Chancellor on brink of second bailout for banks" embedded in its data.
Characteristics of genesis blocks: Cannot reference a previous block, Often contain special messages or data, Establish the initial state of the blockchain, and May include pre-mined coins for distribution.
Each blockchain has its own genesis block. Ethereum genesis block was created on July 30, 2015, and included the initial distribution of ETH from the crowdsale.