Bitcoin is a cryptocurrency based on blockchain technology. As of 24 March 2017 Bitcoin is the largest cryptocurrency by market capitalization.
Although Bitcoin is the largest and first there are also over one hundred other cryptocurrencies with a market capitalization over $1 million.
What is a Cryptocurrency?
A cryptocurrency is a digital currency in which encryption techniques are used to control the generation of new units of the currency as well as verify the transfer of the units of currency.
Digital currencies like Dash, Ether (Ethereum) and Bitcoin are cryptocurrencies.
The “crypto” in cryptocurrency refers to a the use of cryptography for security.
What is a Blockchain?
Cryptocurrencies like Bitcoin use blockchain technology. Each block is a list of transactions which are linked or chained together to record the entire history of transactions for a given cryptocurrency.
A blockchain can be thought of a ledger that keeps track of all transactions for a given cryptocurrency.
Miners work to solve a cryptographic problem in order to add a valid block to the blockchain.
What is a Hard Fork?
Some cryptocurrencies undergo what is known as a hard fork. A hard fork happens when a group of miners choose to adopt a different set of rules than a different group of miners.
The blockchain then forks, or is split in two, with one chain based on a set of new rules, and the other chain following the old rules.
Blocks added by miners following the old set of rules are no longer compatible with the blocks added by miners with the new set of rules.
When this happens, as is the case with Ethereum, there is effectively becomes two currencies where before there was only one.
The largest example of this to date was Ethereum hard forking, creating Ethereum Classic, which followed the old rules, and Ethereum, which followed the new rules.