A blockchain is a growing list of records, called blocks, that are linked using cryptography. Each block contains a cryptographic hash of the previous block, a timestamp, and transaction data. At its most basic level, blockchain is literally just a chain of blocks, but not in the traditional sense of those words. When we say the words “block” and “chain” in this context, we are actually talking about digital information (the “block”) stored in a public database (the “chain”).
Blockchain is a cryptographically secured, time-stamped, public and distributed database of every bitcoin transaction that has ever occurred on the network. "Distributed" here means that the information in the blockchain is broadcast to and recorded by every node in the network. There is no one central database. Any user can refer to this list of transactions and check exactly what how many bitcoins have ever belonged to any specific address at any point in time. This way the system is transparent, double-spending is prevented, and there is no need for a trusted central authority.
The goal of blockchain is to allow digital information to be recorded and distributed, but not edited. That concept can be difficult to wrap our heads around without seeing the technology in action, so let’s take a look at how the earliest application of blockchain technology actually works.
Blockchain technology was first outlined in 1991 by Stuart Haber and W. Scott Stornetta, two researchers who wanted to implement a system where document timestamps could not be tampered with. But it wasn’t until almost two decades later, with the launch of Bitcoin in January 2009, that blockchain had its first real-world application.
