What is Blockchain?
Blockchain is the underlying technology that allows Bitcoin, Altcoins, and Ethereum to operate. At its most general level, a blockchain is a decentralized data structure, but Blockchain can be thought of as an immutable, distributed database that all parties append to and monitor.
A multitude of blockchains exist for different purposes – you can even spin one of your own up via IBM, Microsoft, or Amazon’s Cloud Services. Some being public, like Bitcoin, and other’s being private, like those used in business and consortiums.
A Bit More Technical
We will use the Bitcoin blockchain for example, but keep in mind that not all blockchains are the same.
A blockchain is… well, a chain of blocks. Each block is a group of transactions that is broadcast out to the network when a cryptographic puzzle is solved by one of the network nodes – this is called Mining. The nodes are rewarded bitcoins by solving this puzzle as soon as the entire network comes to a consensus that the node provided a valid answer. This block is connected to the block previous of it, and that to the one previous of it, and so on. This makes a chain-like structure. Editing anything in any of the blocks is easily detectable, and will quickly be rejected by the network. Coursera provides an excellent course by Princeton University, if you’re interested in learning more.