Sometimes a confusion appears between the blockchain type and its consensus mechanism. This is why it is important to understand what consensus is and its role in a blockchain.
The consensus mechanism makes sure all nodes are synchronised with each other and agree on which transactions are legitimate and added to the blockchain.  It makes sure that everyone in the network uses the same blockchain.
In a centralised system, a central administrator has the authority to maintain and update the database.
Consensus assures that the protocol rules are being followed and guarantees that all transactions occur in a trustless way.  There are some specific goals in the Blockchain consensus protocol, such as agreement, collaboration, cooperation, equal rights for each node, and mandatory participation of each node in the consensus process. 
Other consensus mechanisms: