Blockchain is an algorithm and distributed data structure for managing electronic cash without a central administrator among people who know nothing about one another.
Blockchain is a Distributed Ledger Technology (DLT) that was invented to support the Bitcoin cryptocurrency. It monitors and verifies Bitcoin transactions by calling upon a decentralized network of volunteer-run nodes to, in effect, vote on the order in which transactions occur.
Where is this technology used? So this is best known for being the technology behind cryptocurrencies like Bitcoin and Ether (the currency of Ethereum), but blockchain is much more than an instrument of finance. It also serves as a bookkeeping platform or ledger that is incorruptible, enforces transparency, and bypasses censorship.
So where is the data stored? Well! It is in the public blockchain which means any computer running the Bitcoin client. It records every transaction ever made, including any transactions that sent you coins.
Best explanation of Blockchain technology