Explain what is blockchain?

Blockchain is a decade-old technology that incorporates the following characteristics:

It’s a “Peer-to-Peer” (P2P) network based on cryptography, decentralization, and consensus mechanism. Read more about P2P networks in “What’s a peer-to-peer (P2P) network?”.
Blockchain first emerged as the foundation of the Bitcoin network. Bitcoin and similar digital currency networks are all blockchain-based.
Every computer on this network has a record of all of the data in the blockchain. These computers are called “Nodes”.
The duplication of data on all nodes makes blockchain a distributed database, therefore, it’s also known as “Distributed Ledger Technology” (DLT) with no single point of failure. Read more about DLT in “Distributed ledgers definition”.
There is no central authority in a blockchain network. Every node on a public blockchain network has equal authority. There are no central servers. Even if hackers hack or compromise one server, the network remains undisrupted and the database cannot be altered.
Participants use digital signatures to sign their transaction data. This involves modern data encryption technology, thus improving security for sensitive information.
Blockchain uses cryptographic hash functions and a consensus algorithm to secure data on the network.
These data security measures protect against common cyberattacks like phishing, SQL injections, etc. raise the bar above any reachable level for hackers to attack such networks.
Read more about these characteristics in “How to build your own blockchain using Node.js”.

The above is a generic description of a blockchain network used in a real-world cryptocurrency project like Bitcoin. The technology has since evolved a lot, i.e smart contracts introduced by Ethereum, etc.

There are now also permissioned blockchains for enterprise usage such as Hyperledger Fabric and R3 Corda.