What is a blockchain?

Blockchain is a shared, immutable, and transparent ledger of transactions that are secure, yet accessible to everyone at any time. You can think about blockchain as a digital account book that shows that allows everyone to see everyone else's transactions, yet these transactions cannot be changed even by the initiator of the transaction. It is called a network because it connects as many people as possible to this ledger, and allows them to share value with each other in assets native to the network.

The first known and most popular peer-to-peer network using blockchain technology is bitcoin. Bitcoin uses a mathematical algorithm to prevent people from sending the same transaction twice. It would have been possible to do this since it takes time before transactions are verified so a bad actor may decide to spend funds twice taking advantage of the time it takes for verification.

The proof-of-work is basically done by computers trying to guess a nonce or random set of characters that result in some output. Guessing and hashing the correct 256-bit characters results in the creation of a block which makes the process of creating Bitcoin difficult. To further secure transactions new nodes validate stay up to date with all spent transactions from the first to the current block. That way you must have sufficient spent Bitcoins pointing to your public key to spend an equivalent amount.