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INTRODUCTION TO BLOCK CHAIN TECHNOLOGY
Block chain (more commonly referred to as Blockchain), is simply, in common words, a linked list of blocks of data that are chained to each other using secure hashing techniques. Each block contains the cryptographic hash of the previous block, a timestamp , and also the transaction data.

It is a distributed ledger of various transactions that are generally represented as Merkle tree. Any block chain is resistant to modifications/changes in data. According to Merriam-Webster block chain is "an open distributed-ledger that can record transactions between 2 parties in an efficiently and a verifiable and a permanent way. " What it means to a have a decentralized "back end" of an application is, that the data is distributed over many "blocks" of data which are linked to each other. Even today many applications use a centralized database that is connected to the application via routing protocols but the disadvantage here is multiple cross origin requests that are made to the database which sometimes result in heavy traffic (multiple requests to be processed in an instant).

DIFFERENT TYPES OF BOCK CHAINS.
There are many types of block chain configurations that exist for different purposes and goals. They are listed as below:

1)Public block chain.

2)Private block chain.

3)Hybrid block chain.

PUBLIC BLOCK CHAIN.
Public block chains are designed to be fully decentralized. They do not have any access restrictions.Anyone can join the network and send transactions and also participate in validation of consensus protocol.

Some of the largest public block chains are Bitcoin block chain and Ethereum block chain.

PRIVATE BLOCK CHAIN.
Private block chains are also termed as Permissioned block chains as the entry such a block chain is restricted, unlike Public block chain.We cannot join unless invited by the network administrators.

HYBRID BLOCK CHAIN(Dragon chain).
Hybrid block chain combines the benefit of both the Private block chain and Public block chain. This means that it combines the privacy benefits of Private block chain with the security and transparency benefits of Public block chain. This gives various companies using BaaS(Block chain as a service), the flexibility to choose as to what data they want to make public and what data they want to make private.

Now that we have understood what block chain is and various types of the same lets understand briefly what is the underlying Distributed Ledger Technology that governs the working of block chains in reality.

DISTRIBUTED LEDGER TECHNOLOGY.
A block chain is a database, but it is not a centralized data base. What this essentially means is that it does not have a single administrator validating all data that is incoming. Instead, all the participants in the block chain validate the incoming data or in other words undertake the provenance of the data coming into the block chain.

But not every block chain is a distributed ledger. In case of hybrid block chain there is not a single block chain. Each node has its own chain again. So its a complex arrangement of chains. This sort of arrangement makes use of the Interchain technology.

REFERENCES.
https://dragonchain.com/blog/differences-between-public-private-blockchains/.