Essentially, a blockchain is a chain of data blocks that are connected in time chronology using cryptography. Each block in the chain is a record of transactions that had taken place – orders, payments, account tracking, shipping conditions etc.
A new block is only added (usually at regular time intervals eg. 5s, 10min etc.) with general consensus – when all parties with vested interest, in a peer-to-peer network, have agreed to the legitimacy of the transactions recorded.
The updated blockchain is then replicated and an identical copy distributed to all parties involved.
Because all parties have a copy of the updated blockchain and will not allow any existing blocks to be edited/deleted, it becomes near impossible for any single party or hacker to corrupt the blockchain, because he will have to crack the blockchains held by many other parties in order to illegally modify the blockchain.
It’s like all parties are holding on to a copy of the latest physical ledger, so anyone wanting to illegally modify existing entries will have to steal these ledgers, corrupt them and put them back without being detected.
To me, the three key attributes to the blockchain concept are the common verification through consensus, the irrevocability once updated, and the distribution/sharing of the ledger.
The distribution of identical copies of the blockchain means that there is no single point of failure or vulnerability of a traditional single, centralised ledger.
Even if a natural disaster strikes, there are multiple identical copies across geographical areas that can survive the disaster.
And to hack or illegally add/modify entries to the ledger, the perpetrator would need to crack multiple parties that hold the many copies of the blockchain.
The result is a resilient, redundant and comprehensive ledger that all parties can trust, which is why blockchain technology is the basis of many FinTech applications, including cryptocurrencies like Bitcoin.