Bitcoin is a network that runs on a protocol known as the blockchain. A 2008 paper by a person or people calling themselves Satoshi Nakamoto first described both the blockchain and Bitcoin and for a while the two terms were all but synonymous.
The blockchain has since evolved into a separate concept, and thousands of blockchains have been created using similar cryptographic techniques. This history can make the nomenclature confusing. Blockchain sometimes refers to the original, Bitcoin blockchain. At other times it refers to blockchain technology in general, or to any other specific blockchain, such as the one that powers Ethereum.
The basics of blockchain technology are mercifully straightforward. Any given blockchain consists of a single chain of discrete blocks of information, arranged chronologically. In principle this information can be any string of 1s and 0s, meaning it could include emails, contracts, land titles, marriage certificates, or bond trades. In theory, any type of contract between two parties can be established on a blockchain as long as both parties agree on the contract. This takes away any need for a third party to be involved in any contract. This opens a world of possibilities including peer-to-peer financial products, like loans or decentralized savings and checking accounts, where banks or any intermediary is irrelevant.
While Bitcoin's current goal is a store of value as well as a payment system, there is nothing to say that Bitcoin could not be used in such a way in the future, though consensus would need to be reached to add these systems to Bitcoin. The main goal of the Ethereum project is to have a platform where these "smart contracts" can occur, therefore creating a whole realm of decentralized financial products without any middlemen and the fees and potential data breaches that come along with them.
This versatility has caught the eye of governments and private corporations; indeed, some analysts believe that blockchain technology will ultimately be the most impactful aspect of the cryptocurrency craze.
In Bitcoin's case, though, the information on the blockchain is mostly transactions.
Bitcoin is really just a list. Person A sent X bitcoin to person B, who sent Y bitcoin to person C, etc. By tallying these transactions up, everyone knows where individual users stand. It's important to note that these transactions do not necessarily need to be done from human to human.
Anything can access and use the Bitcoin network and your ethnicity, gender, religion, species, or political leaning are completely irrelevant. This creates vast possibilities for the internet of things. In the future, we could see systems where self-driving taxis or uber vehicles have their own blockchain wallets. The car would be sent cryptocurrency from the passenger and would not move until funds are received. The vehicle would be able to assess when it needs fuel and would use its wallet to facilitate a refill.
Another name for a blockchain is a "distributed ledger," which emphasizes the key difference between this technology and a well-kept Word document. Bitcoin's blockchain is distributed, meaning that it is public. Anyone can download it in its entirety or go to any number of sites that parse it. This means that the record is publicly available, but it also means that there are complicated measures in place for updating the blockchain ledger. There is no central authority to keep tabs on all bitcoin transactions, so the participants themselves do so by creating and verifying "blocks" of transaction data.