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Distributed ledger technology.
A blockchain is a digital, distributed ledger. In the same way that banks, stores, and other businesses keep a record of all of their transactions, a blockchain keeps track of all transactions that take place on its network. The distributed portion is important because it means that everyone on the network has a copy of the ledger.
Whenever a transaction occurs, everyone's ledger is updated at the same time. Transactions, once validated, are also immutable because of this distributed nature; it is very difficult and often impossible to change a confirmed transaction.
Transactions on a blockchain are secured through cryptography. Participants have their own private keys that act as a personal digital signature. If the record of a transaction is altered, the signature will become invalid, and the network will recognize this and reject the change.
But what happens if someone tries to cheat by double spending? Because everyone's ledger is updated at the same time, there must be a consensus among the holders of the ledger to validate the transaction. If a majority of people vote against the transaction, it is removed from the ledger and the network does not allow it to go through.
It is also impossible to change one person's ledger without changing the entire network, making it tamperproof. Because of this, it is an effective way to prevent fraud and bad actors from moving money.
Blockchains have a variety of applications that are currently being explored. They can be used to store data about monetary transactions, the ownership of assets, contracts, and even personal information, and its potential uses are much broader, including tracking supply chains, creating secure digital identities, and much more.