Here’s the ELI5 (“Explain it Like I’m 5”) version. You can think of a public key as a school locker and the private key as the locker combination. Teachers, students, and even your crush can insert letters and notes through the opening in your locker. However, the only person that can retrieve the contents of the mailbox is the one that has the unique key. It should be noted, however, that while school locker combinations are kept in the principal’s office, there is no central database that keeps track of a blockchain network’s private keys. If a user misplaces their private key, they will lose access to their Bitcoin wallet, as was the case with this man who made national headlines in December of 2017.
Proof of work does not make attacks by hackers impossible, but it does make them somewhat useless. If a hacker wanted to coordinate an attack on the blockchain, they would need to solve complex computational math problems at 1 in 5.8 trillion odds just like everyone else. The cost of organizing such an attack would almost certainly outweigh the benefits.
Now, if there is no central system, how would everyone in the system get to know that a certain transaction has happened? The network follows the gossip protocol. Think of how gossip spreads. Suppose Alice sent 3 ETH to Bob. The nodes nearest to her will get to know of this, and then they will tell the nodes closest to them, and then they will tell their neighbors, and this will keep on spreading out until everyone knows. Nodes are basically your nosy, annoying relatives.
Health care providers can leverage blockchain to securely store their patients’ medical records. When a medical record is generated and signed, it can be written into the blockchain, which provides patients with the proof and confidence that the record cannot be changed. These personal health records could be encoded and stored on the blockchain with a private key, so that they are only accessible by certain individuals, thereby ensuring privacy
A small class of digital currencies known as privacy coins aims to make blockchain-based transactions untraceable. They do this by beefing up the protocols designed to obscure the identity of the sender and receiver of funds, as well as the dollar amount being sent. Yes, privacy coins have been accused of being a haven for the criminal community. However, most privacy coin and blockchain developers also suggest that this is a minute component of their community, and that nearly all members are legitimate consumers and businesses.
The receiver of the first bitcoin transaction was cypherpunk Hal Finney, who created the first reusable proof-of-work system (RPOW) in 2004. Finney downloaded the bitcoin software on its release date, and on 12 January 2009 received ten bitcoins from Nakamoto. Other early cypherpunk supporters were creators of bitcoin predecessors: Wei Dai, creator of b-money, and Nick Szabo, creator of bit gold. In 2010, the first known commercial transaction using bitcoin occurred when programmer Laszlo Hanyecz bought two Papa John's pizzas for 10,000 bitcoin.