What is a blockchain?

Blockchain: A shared ledger of transactions kept by many computers at once. Changing it afterwards is practically impossible.

A blockchain is a long list of transactions, grouped into blocks. Each new block is linked to the one before it. That forms a chain of blocks.

Thousands of computers around the world keep a copy of that list. Together they check whether new transactions are valid. No single bank or company decides on its own.

To change an old transaction, someone would have to change every block after it and win over most of the computers. In practice that is almost impossible.

Example
You send 50 euros worth of bitcoin to a friend. The transaction goes into a new block. On bitcoin a new block is added about every 10 minutes on average. After that anyone can see the amount was sent, but not who you are, unless your address is linked to you somewhere else.

Watch out
A blockchain transaction usually cannot be reversed. If you send to the wrong address, the money is often gone.

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