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.
In the example above (a "public Blockchain"), there are multiple versions of you as “nodes” on a network acting as executors of transactions and miners simultaneously. Transactions are collected into blocks before being added to the Blockchain. Miners receive a Bitcoin reward based upon the computational time it takes to work out a) whether the transaction is valid and b) what is the correct mathematical key to link to the block of transactions into the correct place in the open ledger. As more transactions are executed, more Bitcoins flow into the virtual money supply. The "reward" miners get will reduces every 4 years until Bitcoin production will eventually cease (although estimates say this won't be until 2140!). Of course, although the original Blockchain was intended to manage Bitcoin, other virtual currencies, such as Ether, can be used.
One obvious hurdle is the adoption of the technology. To deploy blockchain, financial institutions would essentially have to abandon their current networks and start anew. Trying to integrate the current payment networks with blockchain could prove exceptionally challenging -- to the point where some businesses don't even bother trying to do so. It's also still unclear, with the exception of bitcoin (CCY: BTC-USD), the world's most popular cryptocurrency, if any blockchain aside from bitcoin could survive being scaled to handle a lot of transactions.
Say John buys a lemonade from Sandy’s lemonade stand. On John’s copy of the blockchain, he marks that transaction down: “John bought Lemonade from Sandy, $2.” His copy gets spread around town to all the lemonade stands and lemonade buyers, who add this transaction to their own copies. By the time John has finished drinking that lemonade, everyone’s blockchain ledger shows that he bought his lemonade from Sandy for $2.
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.