According to Sunny Ray, Co-founder and President of India's leading bitcoin blockchain company, Unocoin, Blockchain enables two entities that do not know each other to agree that something is true without the need of a third party. As opposed to writing entries into a single sheet of paper, a blockchain is a distributed database that takes a number of inputs and places them into a block.
Each block is then chained to the next block using a cryptographic signature. This allows blockchains to be used as a ledger which is accessible by anyone with permission to do so. If everyone in the process is pre-selected, the ledger is termed 'permissioned'. If the process is open to the whole world, the ledger is called non permissioned.