In simple terms “how the internet is for an email, the Blockchain is for Bitcoin (CryptoCurrencies). So now if i ask you what is internet, then you cannot give a single line answer about what internet is!. In the same way it is hard to define Blockchain in single line.
So the Scope and implementation of the Blockchain technology is wider.
We can describe Blockchain with following points.
Blockchain is a De-Centralised Peer to Peer Network
Blockchain is Distributed Ledger
Blockchain is Secure
Blockchain is immutable
Blockchain is basically a chain of blocks created by validating transactions and adding to the chain. The validated transactions constitute blocks in a blockchain and each block links to previous block using hash pointer. There is something called proof of work in blockchain, which basically is a mechanism used to validate and approve the transactions to be added as a block to the chain. There are various consensus algorithms used to create proof of work in different blockchain based projects.
The hyperledger project is other most popular blockchain based open source projects catering to multiple use cases.
IOTA is other blockchain technology based project targeted for machine to machine transactions and IOT applications but it has neither a block nor chain!