What is Genesis block?

A genesis block is the first block of a block chain. Modern versions of Bitcoin assign it block number 0, though older versions gave it number 1. The genesis block is almost always hardcoded into the software. It is a special case in that it does not reference a previous block, and for Bitcoin and almost all of its derivatives, it produces an unspendable subsidy.
Main network genesis block
Here is a representation of the genesis block[1] as it appeared in a comment in an old version of Bitcoin (line 1613). The first section defines exactly all of the variables necessary to recreate the block. The second section is the block in standard printblock format, which contains shortened versions of the data in the first section.
GetHash() = 0x000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f
hashMerkleRoot = 0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b
txNew.vin[0].scriptSig = 486604799 4 0x736B6E616220726F662074756F6C69616220646E6F63657320666F206B6E697262206E6F20726F6C6C65636E61684320393030322F6E614A2F33302073656D695420656854
txNew.vout[0].nValue = 5000000000
txNew.vout[0].scriptPubKey = 0x5F1DF16B2B704C8A578D0BBAF74D385CDE12C11EE50455F3C438EF4C3FBCF649B6DE611FEAE06279A60939E028A8D65C10B73071A6F16719274855FEB0FD8A6704 OP_CHECKSIG
block.nVersion = 1
block.nTime = 1231006505
block.nBits = 0x1d00ffff
block.nNonce = 2083236893
CBlock(hash=000000000019d6, ver=1, hashPrevBlock=00000000000000, hashMerkleRoot=4a5e1e, nTime=1231006505, nBits=1d00ffff, nNonce=2083236893, vtx=1)
CTransaction(hash=4a5e1e, ver=1, vin.size=1, vout.size=1, nLockTime=0)
CTxIn(COutPoint(000000, -1), coinbase 04ffff001d0104455468652054696d65732030332f4a616e2f32303039204368616e63656c6c6f72206f6e206272696e6b206f66207365636f6e64206261696c6f757420666f722062616e6b73)
CTxOut(nValue=50.00000000, scriptPubKey=0x5F1DF16B2B704C8A578D0B)
vMerkleTree: 4a5e1e

The first 50 BTC block reward went to address 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa, though this reward can't be spent due to a quirk in the way that the genesis block is expressed in the code; it is not known if this was intentional.