What is blockchain?
A blockchain is a decentralized and distributed digital ledger for recording the history of transactions – that cannot be altered. It is the technology that enables the existence of cryptocurrency (among other things). Bitcoin is the name of the best-known cryptocurrency, the one for which blockchain technology was invented.

How does blockchain work?

Public vs Private Blockchain
The sole distinction between public and private blockchain is related to who is allowed to participate in the network, execute the consensus protocol and maintain the shared ledger.

A public blockchain network is completely open and anyone can join and participate in the network. The network typically has an incentivizing mechanism to encourage more participants to join the network. Bitcoin is one of the largest public blockchain networks in production today.

A private blockchain network requires an invitation and must be validated by either the network starter or by a set of rules put in place by the network starter. Once an entity has joined the network, it will play a role in maintaining the blockchain in a decentralized manner. Hyperledger Fabric is an example of a permissioned blockchain framework implementation and one of the Hyperledger projects hosted by The Linux Foundation.

What is a Hard Fork?
A hard fork refers to a situation when a blockchain splits into two separate chains in consequence of the use of two distinct sets of rules trying to govern the system. A hard fork can be implemented to correct important security risks found in older versions of the software, to add new functionality, or to reverse transactions (as in the case with the hard fork to reverse the hack on the DAO in the Ethereum blockchain).