Bitcoin is the first digital currency based on blockchain. It solved the double-spending problem and enabled peer-to-peer transactions on a large scale. Bitcoin was designed to work as a trustless digital currency that would function without government oversight or a central bank.
Bitcoin was developed by Satoshi Nakamoto, whose identity has never been confirmed and has become the subject of great intrigue. Bitcoin builds upon on other cryptographic and digital currency projects that came before it, but its use of blockchain made it more viable. Nakamoto originally released his white paper and open-source software on a cryptography forum. He mined the first block of the Bitcoin chain, called the genesis block in January 2009.
Bitcoins are created as a reward for a process known as mining. One of the core innovations of Bitcoin, is its consensus algorithm, which creates an incentive system that rewards miners for confirming transactions.
Hal Finney, who developed the first reusable proof-of-work system (RPOW), several years before, was the first Bitcoin adopter and received the first bitcoin transaction ever recorded on its blockchain.

Qtum has been described as a hybrid between Ethereum and Bitcoin. Qtum is a smart contracts platform that places a strong focus on value transfers as well as decentralized apps. Qtum enables users to create smart contracts on the platform and conduct cryptocurrency transactions.
Qtum has a Proof-of-Stake (PoS) consensus algorithm. Several decentralized apps are already using the platform.

Ethereum Classic is the original protocol of Ethereum. As a result of the massive hack on the Ethereum-based DAO, in which around 14% of all ETH in circulation were stolen, a hard fork was proposed to return the stolen funds to their owners. This caused an ideological split revolving around the question of changing previous transactions in the blockchain. Some Ethereum holders rejected the hard fork, and decided to keep using the original protocol, based on the principle that the blockchain is immutable, and cannot be changed.