Following Savjee's youtube tutorial on Creating a blockchain with Javascript. The following includes notes, links, and examples from the tutorial and other resources.
a medium article a block chain in 200 lines of code
Genesis block
The first block in a blockchain is known as a genesis block. Modern versions of Bitcoin number it as block 0, thought very early version counted it as block 1.
The genesis block is almost always hardcoded into the software of the applications that utilize its block chain. 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.
For more information on the genesis block of a blockchain, here is a link to bitcoinwiki.
Difficulty
Difficulty is a measure of how difficult it is to find a hash below a given target.
The Bitcoin network has a global block difficulty. Valid blocks must have a hash below this target. Mining pools also have a pool-specific share difficulty setting a lower limit for shares.
For more information and examples on the difficulty of a blockchain, here is a link to bitcoinwiki.