It is a new block storage technology focused on data decentralisation, integrity and immutability. It is made up of a network where each component (node) has a copy of the blockchain.
This innovation was first developed with the appearance of Bitcoin in 2009, which led to the creation of the first cryptocurrency.
For Bitcoin to succeed, it was crucial to design a system to reliably store monetary transactions, analogous to bank records, but without a central authority. The big innovation this brought about was grouping transactions into blocks and linking them cryptographically. In this way, any change in a past block would break the cryptographic signatures of all subsequent blocks. Thus, integrity of the data was achieved.
By replicating this chain of blocks in all the nodes of the network, it also allowed each one of them to verify this integrity, and if it was not correct, reject the blocks. This achieved immutability: past records could not be changed.
Today there are many variations and implementations of this technology. It is widely used in areas where decentralised storage is required and where data integrity plays an important role, such as cryptocurrencies, distributed databases, notary systems, electronic voting, etc.