Process of ensuring everyone's copy of the transaction data are the same.
Proof of Work
Forces the miner to do some work, to use computational power to solve mathematical puzzle before submitting a block to the blockchain.
The goal is to inflict a modest cost on the person wanting to use the service once, but to make it very expensive for someone to use the service thousands or millions of times.
Hard to compute but easy for others to verify.
Wallet
Store public, private key and address.
private key =gen=> public key =gen=> address
Challenges
Double spend problem, Spending the same money twice