Blockchain has come a long way and now used both as permission less and permissioned blockchain by the industries according to their needs.But the main function of blockchain is open source decentralised ledger which records all the transaction which cannot be altered after recording and is visible to all.The transaction are recorded in each block mined by the miners after finding cryptographic solution to a puzzle and after verifying it is added in blockchain.
It is currently based on POW concesus but POS is also suggested but not implemented till date.The immutable foot prints and transaction are also main security features of the blockchain technology.