A Marvel Of Engineering

To accurately verify transactions on a de-centralized network, many significant issues had to be solved. How do you make sure that a Bitcoin being spent hasn't already been spent somewhere else? And how do yo make sure no one will be able to go back and alter the record of transactions that have taken place already?
The blockchain solves these issues by introducing several innovative safeguards. First, when a transaction takes place, details about it are broadcast to the Bitcoin network and checked by full nodes with a complete copy of the blockchain to make sure it is a legitimate Bitcoin transaction and hasn't been spent before already. Shortly after this happens, the transaction is confirmed by a miner, who creates a record of this transaction and the others that have occurred in the last ten minutes, and adds them to the Bitcoin ledger.

History Of Transactions

Transactions are recorded in these ten-minute "blocks" as a way to show when they took place chronologically, and to prevent modifications from being attempted later on. Each new block of transactions that gets added contains information about the last block that was added before it. This shows the order in which the blocks were recorded, and creates a record of when the transactions within them took place. This method of recording transactions in blocks and "chaining" them together is what gives the blockchain its name.

A Permanent Record

This chronological chain of transaction records makes modification of previous records very difficult. The fact that each block has information abut the previous block means that in order to change a transaction record that has already been recorded, the attacker would have to change not only the block the transaction is in, but also the one that comes after it. So as each new block gets added after the transaction in question, it becomes exponentially more difficult to modify it.